Move Extension into extensions namespace

chrome/browser/extensions/extension_host_mac.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_EXTENSION_HOST_MAC_H_
 #define CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_MAC_H_
 #pragma once
 
 #include "chrome/browser/extensions/extension_host.h"
 
+namespace extensions {
+class Extension;
+}
+
 // TODO(mpcomplete): I don't know what this does or if it is needed anymore,
 // now that ExtensionHost is restructured to rely on WebContents.
 class ExtensionHostMac : public ExtensionHost {
  public:
-  ExtensionHostMac(const Extension* extension,
+  ExtensionHostMac(const extensions::Extension* extension,
                    content::SiteInstance* site_instance,
                    const GURL& url, content::ViewType host_type) :
       ExtensionHost(extension, site_instance, url, host_type) {}
   virtual ~ExtensionHostMac();
  private:
   virtual void UnhandledKeyboardEvent(
       const NativeWebKeyboardEvent& event) OVERRIDE;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionHostMac);
 };
 
 #endif  // CHROME_BROWSER_EXTENSIONS_EXTENSION_HOST_MAC_H_