Move preferred scrollbar style preference change listening from renderer to browser, 2 of 3.

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"
+#include "third_party/WebKit/public/web/mac/WebScrollbarTheme.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();
 
+  // Returns the value of +[NSScroller preferredScrollStyle] as expressed
+  // as the blink enum value.
+  static blink::ScrollerStyle GetPreferredScrollerStyle();
+
   static void SendThemeChangeToAllRenderers(
       float initial_button_delay,
       float autoscroll_button_delay,
       bool jump_on_track_click,
+      blink::ScrollerStyle preferred_scroller_style,
       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_