Remove ENABLE_CUSTOM_ELEMENTS compile time flag

Source/core/features.gypi

 #
 # Copyright (C) 2009 Google Inc. All rights reserved.
 # 
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 # 
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 25 matching lines @@
       'ENABLE_BATTERY_STATUS=0',
       'ENABLE_CANVAS_PROXY=1',
       'ENABLE_CHANNEL_MESSAGING=1',
       'ENABLE_CSS3_TEXT=0',
       'ENABLE_CSS3_TEXT_LINE_BREAK=0',
       'ENABLE_CSS_COMPOSITING=0',
       'ENABLE_CSS_DEVICE_ADAPTATION=0',
       'ENABLE_CSS_EXCLUSIONS=1',
       'ENABLE_CSS_IMAGE_SET=1',
       'ENABLE_CSS_REGIONS=1',
-      'ENABLE_CUSTOM_ELEMENTS=1',
       'ENABLE_CUSTOM_SCHEME_HANDLER=0',
       'ENABLE_DIALOG_ELEMENT=1',
       'ENABLE_ENCRYPTED_MEDIA=1',
       'ENABLE_JAVASCRIPT_I18N_API=1',
       'ENABLE_LEGACY_WEB_AUDIO=1',
       'ENABLE_MATHML=0',
       'ENABLE_MEDIA_STREAM=<(enable_webrtc)',
       'ENABLE_RESOLUTION_MEDIA_QUERY=0',
       'ENABLE_RUBY=1',
       'ENABLE_SANDBOX=1',
@@ skipping 106 matching lines @@
         ],
       }],
       ['use_default_render_theme==1', {
         'feature_defines': [
           'ENABLE_DEFAULT_RENDER_THEME=1',
         ],
       }],
     ],
   },
 }