Introduce a field trial to strictly block "blockable" mixed content.

content/public/common/web_preferences.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CONTENT_PUBLIC_COMMON_WEB_PREFERENCES_H_
 #define CONTENT_PUBLIC_COMMON_WEB_PREFERENCES_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 121 matching lines @@
   // If true, taints all <canvas> elements, regardless of origin.
   bool disable_reading_from_canvas;
   // Strict mixed content checking disables both displaying and running insecure
   // mixed content, and disables embedder notifications that such content was
   // requested (thereby preventing user override).
   bool strict_mixed_content_checking;
   // Strict powerful feature restrictions block insecure usage of powerful
   // features (like geolocation) that we haven't yet disabled for the web at
   // large.
   bool strict_powerful_feature_restrictions;
+  // Disallow user opt-in for blockable mixed content.
+  bool strictly_block_blockable_mixed_content;
   bool password_echo_enabled;
   bool should_print_backgrounds;
   bool should_clear_document_background;
   bool enable_scroll_animator;
   bool css_variables_enabled;
   bool touch_enabled;
   // TODO(mustaq): Nuke when the new API is ready
   bool device_supports_touch;
   // TODO(mustaq): Nuke when the new API is ready
   bool device_supports_mouse;
@@ skipping 67 matching lines @@
   // We try to keep the default values the same as the default values in
   // chrome, except for the cases where it would require lots of extra work for
   // the embedder to use the same default value.
   WebPreferences();
   ~WebPreferences();
 };
 
 }  // namespace content
 
 #endif  // CONTENT_PUBLIC_COMMON_WEB_PREFERENCES_H_