The invocation of the `hook_html_head_alter` was removed from Drupal core.  
Issue - [Remove invocation of hook_html_head_alter()](https://www.drupal.org/node/2555069).

D8 has a new hook to achieve it : [hook_page_attachments_alter()](https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_page_attachments_alter/8).

Code from the change record :

    function MYTHEME_page_attachments_alter(array &$page) {
        $viewport = [
          '#type' => 'html_tag',
          '#tag' => 'meta',
          '#attributes' => [
            'name' => 'viewport',
            'content' => 'width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no',
          ],
        ];
        $page['#attached']['html_head'][] = [$viewport, 'viewport'];
      }