Added a runtime flag for the MQ parser

To make sure that the new MediaQueryParser doesn't cause compatibility issues in M35, it was disabled for main thread MQ evaluation in https://codereview.chromium.org/219183002/ .
This CL adds a runtime flag that would enable turning it back on.

BUG=358078

Review URL: https://codereview.chromium.org/221023002

git-svn-id: svn://svn.chromium.org/blink/trunk@170715 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/css/MediaList.cpp

@@ -71,6 +71,9 @@ PassRefPtrWillBeRawPtr<MediaQuerySet> MediaQuerySet::create(const String& mediaS
     if (mediaString.isEmpty())
         return MediaQuerySet::create();
 
+    if (RuntimeEnabledFeatures::mediaQueryParserEnabled())
+        return MediaQueryParser::parse(mediaString);
+
     BisonCSSParser parser(strictCSSParserContext());
     return parser.parseMediaQueryList(mediaString);
 }

third_party/WebKit/Source/platform/RuntimeEnabledFeatures.in

@@ -69,6 +69,7 @@ LangAttributeAwareFormControlUI
 PrefixedEncryptedMedia status=stable
 LocalStorage status=stable
 Media status=stable
+MediaQueryParser status=test
 MediaSource status=stable
 MediaSourceExperimental depends_on=MediaSource, status=experimental
 MediaStream status=stable