Move WebRuntimeEnabledFeatures to platform.

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'ContentDecryptionModuleResult.h',
@@ skipping 334 matching lines @@
       'exported/WebPresentationController.cpp',
       'exported/WebProcessMemoryDump.cpp',
       'exported/WebRTCConfiguration.cpp',
       'exported/WebRTCICECandidate.cpp',
       'exported/WebRTCOfferOptions.cpp',
       'exported/WebRTCSessionDescription.cpp',
       'exported/WebRTCSessionDescriptionRequest.cpp',
       'exported/WebRTCStatsRequest.cpp',
       'exported/WebRTCStatsResponse.cpp',
       'exported/WebRTCVoidRequest.cpp',
+      'exported/WebRuntimeFeatures.cpp',
       'exported/WebScrollbarImpl.cpp',
       'exported/WebScrollbarImpl.h',
       'exported/WebScrollbarThemeClientImpl.cpp',
       'exported/WebScrollbarThemeClientImpl.h',
       'exported/WebScrollbarThemeGeometryNative.cpp',
       'exported/WebScrollbarThemeGeometryNative.h',
       'exported/WebScrollbarThemePainter.cpp',
       'exported/WebSecurityOrigin.cpp',
       'exported/WebServicePortProviderClient.cpp',
       'exported/WebServiceWorkerProviderClient.cpp',
@@ skipping 910 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }