blink: Make WebMainThreadScrollingReason wrap the cc enum.

third_party/WebKit/Source/platform/blink_platform.gypi

 {
   'includes': [
     'platform_generated.gypi',
   ],
   'variables': {
     'platform_files': [
       'AsyncFileSystemCallbacks.h',
       'CalculationValue.h',
       'CheckedInt.h',
       'Clock.cpp',
@@ skipping 824 matching lines @@
       'network/WebSocketHandshakeRequest.cpp',
       'network/WebSocketHandshakeRequest.h',
       'network/WebSocketHandshakeResponse.cpp',
       'network/WebSocketHandshakeResponse.h',
       'plugins/PluginData.cpp',
       'plugins/PluginData.h',
       'plugins/PluginListBuilder.cpp',
       'plugins/PluginListBuilder.h',
       'scheduler/CancellableTaskFactory.cpp',
       'scheduler/CancellableTaskFactory.h',
+      'scroll/MainThreadScrollingReason.h',
       'scroll/ProgrammaticScrollAnimator.cpp',
       'scroll/ProgrammaticScrollAnimator.h',
       'scroll/ScrollAnimatorBase.cpp',
       'scroll/ScrollAnimatorBase.h',
       'scroll/ScrollAnimatorCompositorCoordinator.cpp',
       'scroll/ScrollAnimatorCompositorCoordinator.h',
       'scroll/ScrollAnimator.cpp',
       'scroll/ScrollAnimator.h',
       'scroll/ScrollTypes.h',
       'scroll/ScrollableArea.cpp',
@@ skipping 277 matching lines @@
       ['OS!="mac"',
         {
           'platform_test_files': [
             'scroll/ScrollAnimatorTest.cpp',
           ],
         }
       ],
     ],
   },
 }