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

content/renderer/render_thread_impl.h

 // 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.
 
 #ifndef CONTENT_RENDERER_RENDER_THREAD_IMPL_H_
 #define CONTENT_RENDERER_RENDER_THREAD_IMPL_H_
 
 #include <set>
 #include <string>
 #include <vector>
@@ skipping 362 matching lines @@
   void OnSetZoomLevelForCurrentURL(const std::string& scheme,
                                    const std::string& host,
                                    double zoom_level);
   void OnCreateNewView(const ViewMsg_New_Params& params);
   void OnTransferBitmap(const SkBitmap& bitmap, int resource_id);
   void OnPurgePluginListCache(bool reload_pages);
   void OnNetworkStateChanged(bool online);
   void OnGetAccessibilityTree();
   void OnTempCrashWithData(const GURL& data);
   void OnSetRendererProcessID(base::ProcessId process_id);
-  void OnSetWebKitSharedTimersSuspended(bool suspend);
   void OnMemoryPressure(
       base::MemoryPressureListener::MemoryPressureLevel memory_pressure_level);
+#if defined(OS_ANDROID)
+  void OnSetWebKitSharedTimersSuspended(bool suspend);
+#endif
+#if defined(OS_MACOSX)
+  void OnUpdateScrollbarTheme(float initial_button_delay,
+                              float autoscroll_button_delay,
+                              bool jump_on_track_click,
+                              bool redraw);
+#endif
 
   void IdleHandlerInForegroundTab();
 
   scoped_ptr<WebGraphicsContext3DCommandBufferImpl> CreateOffscreenContext3d();
 
   // These objects live solely on the render thread.
   scoped_ptr<AppCacheDispatcher> appcache_dispatcher_;
   scoped_ptr<DomStorageDispatcher> dom_storage_dispatcher_;
   scoped_ptr<IndexedDBDispatcher> main_thread_indexed_db_dispatcher_;
   scoped_ptr<RendererWebKitPlatformSupportImpl> webkit_platform_support_;
@@ skipping 97 matching lines @@
   scoped_ptr<GamepadSharedMemoryReader> gamepad_shared_memory_reader_;
 
   base::ProcessId renderer_process_id_;
 
   DISALLOW_COPY_AND_ASSIGN(RenderThreadImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_RENDERER_RENDER_THREAD_IMPL_H_