Remove dependency on BrowserThread::FILE from Sync code

chrome/browser/sync/chrome_sync_client.cc

 // Copyright 2015 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/sync/chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/memory/ptr_util.h"
 #include "base/path_service.h"
 #include "base/syslog_logging.h"
 #include "build/build_config.h"
 #include "chrome/browser/autofill/personal_data_manager_factory.h"
 #include "chrome/browser/bookmarks/bookmark_model_factory.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/dom_distiller/dom_distiller_service_factory.h"
+#include "chrome/browser/extensions/api/storage/backend_task_runner.h"
 #include "chrome/browser/favicon/favicon_service_factory.h"
 #include "chrome/browser/history/history_service_factory.h"
 #include "chrome/browser/invalidation/profile_invalidation_provider_factory.h"
 #include "chrome/browser/password_manager/password_store_factory.h"
 #include "chrome/browser/prefs/pref_service_syncable_util.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/browser/search_engines/template_url_service_factory.h"
 #include "chrome/browser/signin/profile_oauth2_token_service_factory.h"
 #include "chrome/browser/sync/glue/sync_start_util.h"
@@ skipping 500 matching lines @@
 }
 
 scoped_refptr<syncer::ModelSafeWorker>
 ChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   switch (group) {
     case syncer::GROUP_DB:
       return new syncer::SequencedModelWorker(
           BrowserThread::GetTaskRunnerForThread(BrowserThread::DB),
           syncer::GROUP_DB);
+    // TODO(stanisc): crbug.com/731903: Rename GROUP_FILE to reflect that it is
+    // used only for app and extension settings.
     case syncer::GROUP_FILE:
+#if BUILDFLAG(ENABLE_EXTENSIONS)
       return new syncer::SequencedModelWorker(
-          BrowserThread::GetTaskRunnerForThread(BrowserThread::FILE),
-          syncer::GROUP_FILE);
+          extensions::GetBackendTaskRunner(), syncer::GROUP_FILE);
+#else
+      return nullptr;
+#endif  // BUILDFLAG(ENABLE_EXTENSIONS)
     case syncer::GROUP_UI:
       return new syncer::UIModelWorker(
           BrowserThread::GetTaskRunnerForThread(BrowserThread::UI));
     case syncer::GROUP_PASSIVE:
       return new syncer::PassiveModelWorker();
     case syncer::GROUP_HISTORY: {
       history::HistoryService* history_service = GetHistoryService();
       if (!history_service)
         return nullptr;
       return new HistoryModelWorker(
@@ skipping 165 matching lines @@
   sync_service->RegisterDataTypeController(
       base::MakeUnique<SupervisedUserSyncDataTypeController>(
           syncer::SUPERVISED_USER_SETTINGS, error_callback, this, profile_));
   sync_service->RegisterDataTypeController(
       base::MakeUnique<SupervisedUserSyncDataTypeController>(
           syncer::SUPERVISED_USER_WHITELISTS, error_callback, this, profile_));
 #endif  // BUILDFLAG(ENABLE_SUPERVISED_USERS)
 }
 
 }  // namespace browser_sync