Move the CSS Device Adaptation @viewport rule support behind a runtime 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 16 matching lines @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #
 
 {
   # The following defines turn WebKit features on and off.
   'variables': {
     'feature_defines': [
       'ENABLE_CANVAS_USES_MAILBOX=1',
       'ENABLE_CSS3_TEXT=0',
-      'ENABLE_CSS_DEVICE_ADAPTATION=0',
       'ENABLE_CSS_EXCLUSIONS=1',
       'ENABLE_CSS_REGIONS=1',
       'ENABLE_CUSTOM_SCHEME_HANDLER=0',
       'ENABLE_ENCRYPTED_MEDIA_V2=1',
       'ENABLE_GRAPHICS_CONTEXT_ANNOTATIONS=<(enable_graphics_context_annotations)',
       'ENABLE_SVG_FONTS=1',
       'ENABLE_TOUCH_ICON_LOADING=<(enable_touch_icon_loading)',
       'ENABLE_XHR_TIMEOUT=0',
       # WTF_USE_DYNAMIC_ANNOTATIONS=1 may be defined in build/common.gypi
       # We can't define it here because it should be present only
@@ skipping 80 matching lines @@
         ],
       }],
       ['use_default_render_theme==1', {
         'feature_defines': [
           'ENABLE_DEFAULT_RENDER_THEME=1',
         ],
       }],
     ],
   },
 }