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

content/content_browser.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 {
   'dependencies': [
     'browser/speech/proto/speech_proto.gyp:speech_proto',
     '../base/base.gyp:base_static',
     '../crypto/crypto.gyp:crypto',
     '../google_apis/google_apis.gyp:google_apis',
@@ skipping 1115 matching lines @@
     'browser/streams/stream_url_request_job.h',
     'browser/streams/stream_write_observer.h',
     'browser/storage_partition_impl.cc',
     'browser/storage_partition_impl.h',
     'browser/storage_partition_impl_map.cc',
     'browser/storage_partition_impl_map.h',
     'browser/system_message_window_win.cc',
     'browser/system_message_window_win.h',
     'browser/tcmalloc_internals_request_job.cc',
     'browser/tcmalloc_internals_request_job.h',
+    'browser/theme_helper_mac.mm',
+    'browser/theme_helper_mac.h',
     'browser/tracing/trace_controller_impl.cc',
     'browser/tracing/trace_controller_impl.h',
     'browser/tracing/trace_message_filter.cc',
     'browser/tracing/trace_message_filter.h',
     'browser/tracing/trace_subscriber_stdio.cc',
     'browser/tracing/trace_subscriber_stdio.h',
     'browser/tracing/tracing_controller_impl.cc',
     'browser/tracing/tracing_controller_impl.h',
     'browser/tracing/tracing_ui.cc',
     'browser/tracing/tracing_ui.h',
@@ skipping 417 matching lines @@
         ['exclude', '^browser/speech/input_tag_speech_dispatcher_host\\.(cc|h)$'],
       ],
     }, {  # input_speech==1
       'dependencies': [
         '../third_party/flac/flac.gyp:libflac',
         '../third_party/speex/speex.gyp:libspeex',
       ],
     }],
   ],
 }