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

chrome/renderer/extensions/renderer_extension_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_RENDERER_EXTENSION_BINDINGS_H_
 #define CHROME_RENDERER_EXTENSIONS_RENDERER_EXTENSION_BINDINGS_H_
 #pragma once
 
-#include "v8/include/v8.h"
-
 #include <string>
 
 class ExtensionRendererContext;
 class GURL;
 class RenderView;
 
+namespace v8 {
+class Extension;
+}
+
 // This class adds extension-related javascript bindings to a renderer.  It is
 // used by both web renderers and extension processes.
 class RendererExtensionBindings {
  public:
   // Name of extension, for dependencies.
   static const char* kName;
 
   // Creates an instance of the extension.
   static v8::Extension* Get(ExtensionRendererContext* context);
 };
 
 #endif  // CHROME_RENDERER_EXTENSIONS_RENDERER_EXTENSION_BINDINGS_H_