I'm not sure I understood the question... it is very simple.
You don't need contextual filter, you need to add a relationship with Flag by current user.
Click on relationship a look for Flags: User's flagged content, then select the type of flag and here we go, you got a page or a block with the content flagged by the current user.
If you are using drupal 7 import this view to see if it's what you are looking for
$view = new view;
$view->name = 'User Follow';
$view->description = '';
$view->tag = 'flag';
$view->base_table = 'node';
$view->human_name = '';
$view->core = 0;
$view->api_version = '3.0';
$view->disabled = FALSE; /* Edit this to true to make a default view disabled initially */
/* Display: Defaults */
$handler = $view->new_display('default', 'Defaults', 'default');
$handler->display->display_options['title'] = 'User bookmarks';
$handler->display->display_options['access']['type'] = 'role';
$handler->display->display_options['access']['role'] = array(
2 => '2',
);
$handler->display->display_options['cache']['type'] = 'none';
$handler->display->display_options['query']['type'] = 'views_query';
$handler->display->display_options['query']['options']['query_comment'] = FALSE;
$handler->display->display_options['exposed_form']['type'] = 'basic';
$handler->display->display_options['pager']['type'] = 'none';
$handler->display->display_options['style_plugin'] = 'table';
$handler->display->display_options['style_options']['columns'] = array(
'type' => 'type',
'title' => 'title',
'name' => 'name',
'comment_count' => 'comment_count',
'last_comment_timestamp' => 'last_comment_timestamp',
);
$handler->display->display_options['style_options']['default'] = 'last_comment_timestamp';
$handler->display->display_options['style_options']['info'] = array(
'type' => array(
'sortable' => TRUE,
),
'title' => array(
'sortable' => TRUE,
),
'name' => array(
'sortable' => TRUE,
),
'comment_count' => array(
'sortable' => TRUE,
),
'last_comment_timestamp' => array(
'sortable' => TRUE,
),
);
$handler->display->display_options['style_options']['override'] = FALSE;
$handler->display->display_options['style_options']['sticky'] = 1;
$handler->display->display_options['style_options']['order'] = 'desc';
/* No results behavior: Global: Text area */
$handler->display->display_options['empty']['text']['id'] = 'area';
$handler->display->display_options['empty']['text']['table'] = 'views';
$handler->display->display_options['empty']['text']['field'] = 'area';
$handler->display->display_options['empty']['text']['empty'] = FALSE;
$handler->display->display_options['empty']['text']['content'] = 'This user has not yet bookmarked any content.';
$handler->display->display_options['empty']['text']['format'] = 'plain_text';
/* Relationship: Flags: bookmarks */
$handler->display->display_options['relationships']['flag_content_rel']['id'] = 'flag_content_rel';
$handler->display->display_options['relationships']['flag_content_rel']['table'] = 'node';
$handler->display->display_options['relationships']['flag_content_rel']['field'] = 'flag_content_rel';
$handler->display->display_options['relationships']['flag_content_rel']['label'] = 'bookmarks';
$handler->display->display_options['relationships']['flag_content_rel']['flag'] = 'bookmarks';
$handler->display->display_options['relationships']['flag_content_rel']['user_scope'] = 'any';
/* Relationship: Content: Author */
$handler->display->display_options['relationships']['uid_1']['id'] = 'uid_1';
$handler->display->display_options['relationships']['uid_1']['table'] = 'node';
$handler->display->display_options['relationships']['uid_1']['field'] = 'uid';
$handler->display->display_options['relationships']['uid_1']['required'] = 0;
/* Relationship: Flags: User */
$handler->display->display_options['relationships']['uid']['id'] = 'uid';
$handler->display->display_options['relationships']['uid']['table'] = 'flag_content';
$handler->display->display_options['relationships']['uid']['field'] = 'uid';
$handler->display->display_options['relationships']['uid']['relationship'] = 'flag_content_rel';
$handler->display->display_options['relationships']['uid']['label'] = 'bookmarks_user';
$handler->display->display_options['relationships']['uid']['required'] = 1;
/* Field: Content: Type */
$handler->display->display_options['fields']['type']['id'] = 'type';
$handler->display->display_options['fields']['type']['table'] = 'node';
$handler->display->display_options['fields']['type']['field'] = 'type';
/* Field: Content: Title */
$handler->display->display_options['fields']['title']['id'] = 'title';
$handler->display->display_options['fields']['title']['table'] = 'node';
$handler->display->display_options['fields']['title']['field'] = 'title';
$handler->display->display_options['fields']['title']['link_to_node'] = 1;
/* Field: User: Name */
$handler->display->display_options['fields']['name']['id'] = 'name';
$handler->display->display_options['fields']['name']['table'] = 'users';
$handler->display->display_options['fields']['name']['field'] = 'name';
$handler->display->display_options['fields']['name']['relationship'] = 'uid_1';
$handler->display->display_options['fields']['name']['label'] = 'Author';
$handler->display->display_options['fields']['name']['link_to_user'] = 1;
/* Field: Content: Comment count */
$handler->display->display_options['fields']['comment_count']['id'] = 'comment_count';
$handler->display->display_options['fields']['comment_count']['table'] = 'node_comment_statistics';
$handler->display->display_options['fields']['comment_count']['field'] = 'comment_count';
$handler->display->display_options['fields']['comment_count']['label'] = 'Replies';
/* Field: Content: Last comment time */
$handler->display->display_options['fields']['last_comment_timestamp']['id'] = 'last_comment_timestamp';
$handler->display->display_options['fields']['last_comment_timestamp']['table'] = 'node_comment_statistics';
$handler->display->display_options['fields']['last_comment_timestamp']['field'] = 'last_comment_timestamp';
$handler->display->display_options['fields']['last_comment_timestamp']['label'] = 'Last Post';
/* Contextual filter: User: Uid */
$handler->display->display_options['arguments']['uid']['id'] = 'uid';
$handler->display->display_options['arguments']['uid']['table'] = 'users';
$handler->display->display_options['arguments']['uid']['field'] = 'uid';
$handler->display->display_options['arguments']['uid']['relationship'] = 'uid';
$handler->display->display_options['arguments']['uid']['default_action'] = 'empty';
$handler->display->display_options['arguments']['uid']['exception']['title_enable'] = 1;
$handler->display->display_options['arguments']['uid']['title_enable'] = 1;
$handler->display->display_options['arguments']['uid']['title'] = '%1\'s bookmarks';
$handler->display->display_options['arguments']['uid']['default_argument_type'] = 'fixed';
$handler->display->display_options['arguments']['uid']['summary']['format'] = 'default_summary';
$handler->display->display_options['arguments']['uid']['specify_validation'] = 1;
$handler->display->display_options['arguments']['uid']['break_phrase'] = 0;
$handler->display->display_options['arguments']['uid']['not'] = 0;
/* Filter criterion: Content: Published */
$handler->display->display_options['filters']['status']['id'] = 'status';
$handler->display->display_options['filters']['status']['table'] = 'node';
$handler->display->display_options['filters']['status']['field'] = 'status';
$handler->display->display_options['filters']['status']['value'] = 1;
$handler->display->display_options['filters']['status']['expose']['operator'] = FALSE;
/* Display: Page */
$handler = $view->new_display('page', 'Page', 'page');
$handler->display->display_options['path'] = 'user/%/bookmarks';
$handler->display->display_options['menu']['title'] = 'Bookmarks';
$handler->display->display_options['menu']['weight'] = '0';