Have Blink be told of scrollbar changes rather than paying attention itself (Blink side part 2).

Source/web/mac/WebScrollbarTheme.cpp

 /*
  * Copyright (C) 2013 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 23 matching lines @@
 #include "core/platform/ScrollbarThemeMacCommon.h"
 
 using namespace WebCore;
 
 namespace blink {
 
 void WebScrollbarTheme::updateScrollbars(
     float initialButtonDelay, float autoscrollButtonDelay,
     bool jumpOnTrackClick, bool redraw)
 {
+    ScrollbarTheme* theme = ScrollbarTheme::theme();
+    if (theme->isMockTheme())
+        return;
+
     static_cast<ScrollbarThemeMacCommon*>(ScrollbarTheme::theme())->preferencesChanged(
         initialButtonDelay, autoscrollButtonDelay, jumpOnTrackClick, redraw);
 }
 
 } // namespace blink