Add CSS variables feature flag to chrome://flags.

webkit/glue/webpreferences.h

 // Copyright (c) 2012 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.
 //
 // A struct for managing webkit's settings.
 //
 // Adding new values to this class probably involves updating
 // WebKit::WebSettings, content/common/view_messages.h, browser/tab_contents/
 // render_view_host_delegate_helper.cc, and browser/profiles/profile.cc.
 
@@ skipping 102 matching lines @@
   bool memory_info_enabled;
   bool fullscreen_enabled;
   bool allow_displaying_insecure_content;
   bool allow_running_insecure_content;
   bool password_echo_enabled;
   bool should_print_backgrounds;
   bool enable_scroll_animator;
   bool visual_word_movement_enabled;
   bool css_regions_enabled;
   bool css_shaders_enabled;
+  bool css_variables_enabled;
   bool device_supports_touch;
   bool device_supports_mouse;
 #if !defined(WEBCOMPOSITOR_OWNS_SETTINGS)
   bool threaded_animation_enabled;
   bool per_tile_painting_enabled;
   bool partial_swap_enabled;
 #endif
   int default_tile_width;
   int default_tile_height;
   int max_untiled_layer_width;
@@ skipping 14 matching lines @@
   // the embedder to use the same default value.
   WebPreferences();
   ~WebPreferences();
 
   void Apply(WebKit::WebView* web_view) const;
 };
 
 }  // namespace webkit_glue
 
 #endif  // WEBKIT_GLUE_WEBPREFERENCES_H__