Move Extension into extensions namespace

chrome/browser/extensions/api/runtime/runtime_api.h

 // Copyright (c) 2012 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_BROWSER_EXTENSIONS_API_RUNTIME_RUNTIME_API_H_
 #define CHROME_BROWSER_EXTENSIONS_API_RUNTIME_RUNTIME_API_H_
 #pragma once
 
 #include "chrome/browser/extensions/extension_function.h"
 
-class Extension;
 class ExtensionHost;
 class Profile;
 
 namespace extensions {
 
+class Extension;
+
 class RuntimeEventRouter {
  public:
   // Dispatches the onInstalled event to the given extension.
   static void DispatchOnInstalledEvent(Profile* profile,
                                        const std::string& extension_id);
 };
 
 class RuntimeGetBackgroundPageFunction : public AsyncExtensionFunction {
  protected:
   virtual bool RunImpl() OVERRIDE;
   DECLARE_EXTENSION_FUNCTION_NAME("experimental.runtime.getBackgroundPage");
  private:
   void OnPageLoaded(ExtensionHost*);
 };
 
 }  // namespace extensions
 
 #endif  // CHROME_BROWSER_EXTENSIONS_API_RUNTIME_RUNTIME_API_H_