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

content/renderer/render_thread_impl.h

Index: content/renderer/render_thread_impl.h
diff --git a/content/renderer/render_thread_impl.h b/content/renderer/render_thread_impl.h
index 7492b92e4b4f36335820c844bcc071a11279b1d1..a407a53115ab9e40e7c68cd5b02ea200d184f889 100644
--- a/content/renderer/render_thread_impl.h
+++ b/content/renderer/render_thread_impl.h
@@ -380,9 +380,17 @@ class CONTENT_EXPORT RenderThreadImpl : public RenderThread,
   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();