Add chrome/browser/apps/BUILD.gn and move Chrome-specific code there

apps/saved_files_service_factory.h

 // Copyright 2013 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 APPS_SAVED_FILES_SERVICE_FACTORY_H_
 #define APPS_SAVED_FILES_SERVICE_FACTORY_H_
 
 #include "base/memory/singleton.h"
 #include "components/keyed_service/content/browser_context_keyed_service_factory.h"
 
 namespace content {
 class BrowserContext;
 }
 
 namespace apps {
 
 class SavedFilesService;
 
 // BrowserContextKeyedServiceFactory for SavedFilesService.
 class SavedFilesServiceFactory : public BrowserContextKeyedServiceFactory {
  public:
   static SavedFilesService* GetForBrowserContext(
       content::BrowserContext* context);
 
+  static SavedFilesService* GetForBrowserContextIfExists(
+      content::BrowserContext* context);
+
   static SavedFilesServiceFactory* GetInstance();
 
  private:
   SavedFilesServiceFactory();
   ~SavedFilesServiceFactory() override;
   friend struct base::DefaultSingletonTraits<SavedFilesServiceFactory>;
 
   KeyedService* BuildServiceInstanceFor(
       content::BrowserContext* context) const override;
 };
 
 }  // namespace apps
 
 #endif  // APPS_SAVED_FILES_SERVICE_FACTORY_H_