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

chrome/renderer/extensions/js_only_v8_extensions.cc

 // 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.
 
 #include "chrome/renderer/extensions/js_only_v8_extensions.h"
 
 #include "chrome/renderer/extensions/extension_base.h"
 #include "grit/renderer_resources.h"
+#include "v8/include/v8.h"
 
 // JsonSchemaJsV8Extension
 const char* JsonSchemaJsV8Extension::kName = "chrome/jsonschema";
 v8::Extension* JsonSchemaJsV8Extension::Get() {
   static v8::Extension* extension = new ExtensionBase(
       kName, ExtensionBase::GetStringResource(IDR_JSON_SCHEMA_JS), 0, NULL,
       NULL);
   return extension;
 }
 
 // ExtensionApiTestV8Extension
 const char* ExtensionApiTestV8Extension::kName = "chrome/extensionapitest";
 v8::Extension* ExtensionApiTestV8Extension::Get() {
   static v8::Extension* extension = new ExtensionBase(
       kName, ExtensionBase::GetStringResource(IDR_EXTENSION_APITEST_JS), 0,
       NULL, NULL);
   return extension;
 }