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

content/browser/theme_helper_mac.h

+// Copyright 2013 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_BROWSER_THEME_HELPER_MAC_H_
+#define CONTENT_BROWSER_THEME_HELPER_MAC_H_
+
+#include "base/memory/singleton.h"
+#include "content/public/browser/notification_observer.h"
+#include "content/public/browser/notification_registrar.h"
+
+namespace content {
+
+class ThemeHelperMac : public NotificationObserver {
+ public:
+  // Return pointer to the singleton instance for the current process, or NULL
+  // if none.
+  static ThemeHelperMac* GetInstance();
+
+  static void SendThemeChangeToAllRenderers(
+      float initial_button_delay,
+      float autoscroll_button_delay,
+      bool jump_on_track_click,
+      bool redraw);
+
+ private:
+  friend struct DefaultSingletonTraits<ThemeHelperMac>;
+
+  ThemeHelperMac();
+  virtual ~ThemeHelperMac();
+
+  // Overridden from NotificationObserver:
+  virtual void Observe(int type,
+                       const NotificationSource& source,
+                       const NotificationDetails& details) OVERRIDE;
+
+  NotificationRegistrar registrar_;
+
+  DISALLOW_COPY_AND_ASSIGN(ThemeHelperMac);
+};
+
+}  // namespace content
+
+#endif  // CONTENT_BROWSER_THEME_HELPER_MAC_H_