Forward declare v8::Extension in chrome/renderer/extensions/*.

chrome/renderer/extensions/event_bindings.h

 // Copyright (c) 2011 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 CHROME_RENDERER_EXTENSIONS_EVENT_BINDINGS_H_
 #define CHROME_RENDERER_EXTENSIONS_EVENT_BINDINGS_H_
 #pragma once
 
-#include "v8/include/v8.h"
-
 class ExtensionRendererContext;
 class RenderThreadBase;
 
+namespace v8 {
+class Extension;
+}
+
 // This class deals with the javascript bindings related to Event objects.
 class EventBindings {
  public:
   static const char* kName;  // The v8::Extension name, for dependencies.
 
   static v8::Extension* Get(ExtensionRendererContext* context);
 
   // Allow RenderThread to be mocked out.
   static void SetRenderThread(RenderThreadBase* thread);
   static RenderThreadBase* GetRenderThread();
 };
 
 #endif  // CHROME_RENDERER_EXTENSIONS_EVENT_BINDINGS_H_