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

chrome/renderer/extensions/js_only_v8_extensions.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_JS_ONLY_V8_EXTENSIONS_H_
 #define CHROME_RENDERER_EXTENSIONS_JS_ONLY_V8_EXTENSIONS_H_
 #pragma once
 
-#include "v8/include/v8.h"
+namespace v8 {
+class Extension;
+}
 
 // This file contains various V8 Extensions that are JavaScript only, and
 // don't have any C++ native functions.
 
 class JsonSchemaJsV8Extension {
  public:
   static const char* kName;
   static v8::Extension* Get();
 };
 
 class ExtensionApiTestV8Extension {
  public:
   static const char* kName;
   static v8::Extension* Get();
 };
 
 #endif  // CHROME_RENDERER_EXTENSIONS_JS_ONLY_V8_EXTENSIONS_H_