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

Source/web/web.gyp

 #
 # Copyright (C) 2011 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 187 matching lines @@
                     ],
                     'link_settings': {
                         'libraries': [
                             '$(SDKROOT)/System/Library/Frameworks/Accelerate.framework',
                             '$(SDKROOT)/System/Library/Frameworks/OpenGL.framework',
                         ],
                     },
                 }, { # else: OS!="mac"
                     'sources/': [
                         ['exclude', 'WebInputEventFactoryMac.mm$'],
+                        ['exclude', 'mac/WebScrollbarTheme.cpp$'],
                     ],
                 }],
                 ['OS=="win"', {
                     'include_dirs': [
                         '../../public/web/win',
                     ],
                 }, { # else: OS!="win"
                     'sources/': [
                         ['exclude', 'WebInputEventFactoryWin.cpp$'],
                     ],
@@ skipping 158 matching lines @@
         ['clang==1', {
             'target_defaults': {
                 'cflags': ['-Wglobal-constructors'],
                 'xcode_settings': {
                     'WARNING_CFLAGS': ['-Wglobal-constructors'],
                 },
             },
         }],
     ], # conditions
 }