Have Blink be told of scrollbar changes rather than paying attention itself (Blink side).

Source/web/web.gypi

 {
   'variables': {
     'web_files': [
       'ApplicationCacheHost.cpp',
       'ApplicationCacheHostInternal.h',
       'AssertMatchingEnums.cpp',
       'AssociatedURLLoader.cpp',
       'AssociatedURLLoader.h',
       'AutofillPopupMenuClient.cpp',
       'AutofillPopupMenuClient.h',
@@ skipping 251 matching lines @@
       'WorkerAllowMainThreadBridgeBase.cpp',
       'WorkerAllowMainThreadBridgeBase.h',
       'WorkerFileSystemClient.cpp',
       'WorkerFileSystemClient.h',
       'WorkerPermissionClient.cpp',
       'WorkerPermissionClient.h',
       'default/WebRenderTheme.cpp',
       'linux/WebFontInfo.cpp',
       'linux/WebFontRenderStyle.cpp',
       'linux/WebFontRendering.cpp',
+      'mac/WebScrollbarTheme.cpp',
       'mac/WebSubstringUtil.mm',
       'painting/ContinuousPainter.cpp',
       'painting/ContinuousPainter.h',
       'painting/PaintAggregator.cpp',
       'painting/PaintAggregator.h',
     ],
     'web_unittest_files': [
       'tests/AssociatedURLLoaderTest.cpp',
       'tests/ChromeClientImplTest.cpp',
       'tests/CompositedLayerMappingTest.cpp',
@@ skipping 84 matching lines @@
         {
           'web_unittest_files': [
             'tests/KeyCodeConversionTestGtk.cpp',
             'tests/WebInputEventFactoryTestGtk.cpp',
           ],
         }
       ],
     ],
   },
 }