Create a new chrome/browser/apps folder and move PlatformAppService there.

chrome/browser/platform_apps/platform_app_service.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_PLATFORM_APP_SERVICE_H_
-#define CHROME_BROWSER_EXTENSIONS_PLATFORM_APP_SERVICE_H_
+#ifndef CHROME_BROWSER_PLATFORM_APPS_PLATFORM_APP_SERVICE_H_
+#define CHROME_BROWSER_PLATFORM_APPS_PLATFORM_APP_SERVICE_H_
 
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_keyed_service.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 
-namespace extensions {
+namespace platform_apps {
 
 // The PlatformAppService provides services for platform apps, such as
 // keeping the browser process alive while they are running.
 // TODO(benwells): Merge functionality of ShellWindowRegistry into this class.
 class PlatformAppService : public ProfileKeyedService,
                            public content::NotificationObserver {
  public:
   explicit PlatformAppService(Profile* profile);
   virtual ~PlatformAppService();
 
  private:
   // content::NotificationObserver:
   virtual void Observe(int type,
                        const content::NotificationSource& source,
                        const content::NotificationDetails& details) OVERRIDE;
 
   content::NotificationRegistrar registrar_;
 };
 
-}  // namespace extensions
+}  // namespace platform_apps
 
-#endif  // CHROME_BROWSER_EXTENSIONS_PLATFORM_APP_SERVICE_H_
+#endif  // CHROME_BROWSER_PLATFORM_APPS_PLATFORM_APP_SERVICE_H_