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

chrome/browser/platform_apps/platform_app_service.cc

 // 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.
 
-#include "chrome/browser/extensions/platform_app_service.h"
+#include "chrome/browser/platform_apps/platform_app_service.h"
 
 #include "chrome/browser/extensions/extension_host.h"
 #include "chrome/common/chrome_notification_types.h"
 #include "chrome/common/extensions/extension.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/notification_details.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
 
-namespace extensions {
+namespace platform_apps {
 
 PlatformAppService::PlatformAppService(Profile* profile) {
   registrar_.Add(this, chrome::NOTIFICATION_EXTENSION_HOST_CREATED,
                  content::Source<content::BrowserContext>(profile));
 }
 
 PlatformAppService::~PlatformAppService() {}
 
 void PlatformAppService::Observe(int type,
                                   const content::NotificationSource& source,
                                   const content::NotificationDetails& details) {
   switch (type) {
     case chrome::NOTIFICATION_EXTENSION_HOST_CREATED: {
-      ExtensionHost* host = content::Details<ExtensionHost>(details).ptr();
+      extensions::ExtensionHost* host =
+          content::Details<extensions::ExtensionHost>(details).ptr();
       if (host->extension()->is_platform_app())
         host->SetKeepsBrowserProcessAlive();
       break;
     }
     default:
       NOTREACHED();
     }
 }
 
-}  // namespace extensions
+}  // namespace platform_apps