Remove the ENABLE_IFRAME_SEAMLESS compile-time flag.

Source/WebKit/chromium/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 56 matching lines @@
       'ENABLE_DRAGGABLE_REGION=1',
       'ENABLE_ENCRYPTED_MEDIA=1',
       'ENABLE_FILE_SYSTEM=1',
       'ENABLE_FILTERS=1',
       'ENABLE_FONT_LOAD_EVENTS=1',
       'ENABLE_FULLSCREEN_API=1',
       'ENABLE_GAMEPAD=1',
       'ENABLE_GEOLOCATION=1',
       'ENABLE_GESTURE_EVENTS=1',
       'ENABLE_ICONDATABASE=0',
-      'ENABLE_IFRAME_SEAMLESS=1',
       'ENABLE_INDEXED_DATABASE=1',
       'ENABLE_INPUT_TYPE_DATE=1',
       'ENABLE_INPUT_TYPE_DATETIMELOCAL=1',
       'ENABLE_INPUT_TYPE_MONTH=1',
       'ENABLE_INPUT_TYPE_TIME=1',
       'ENABLE_JAVASCRIPT_DEBUGGER=1',
       'ENABLE_JAVASCRIPT_I18N_API=1',
       'ENABLE_LEGACY_CSS_VENDOR_PREFIXES=0',
       'ENABLE_LEGACY_VIEWPORT_ADAPTION=1',
       'ENABLE_LEGACY_VENDOR_PREFIXES=0',
@@ skipping 165 matching lines @@
         ],
       }],
       ['use_default_render_theme==1', {
         'feature_defines': [
           'ENABLE_DEFAULT_RENDER_THEME=1',
         ],
       }],
     ],
   },
 }