Remove dependency on BrowserThread::FILE from Sync code

ios/chrome/browser/sync/ios_chrome_sync_client.mm

 // 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 "ios/chrome/browser/sync/ios_chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 365 matching lines @@
 
 scoped_refptr<syncer::ModelSafeWorker>
 IOSChromeSyncClient::CreateModelWorkerForGroup(syncer::ModelSafeGroup group) {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   switch (group) {
     case syncer::GROUP_DB:
       return new syncer::SequencedModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::DB),
           syncer::GROUP_DB);
     case syncer::GROUP_FILE:
-      return new syncer::SequencedModelWorker(
-          web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE),
-          syncer::GROUP_FILE);
+      // Not supported on iOS.
+      return nullptr;
     case syncer::GROUP_UI:
       return new syncer::UIModelWorker(
           web::WebThread::GetTaskRunnerForThread(web::WebThread::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 browser_sync::HistoryModelWorker(
@@ skipping 33 matching lines @@
         IOSChromeProfileSyncServiceFactory::GetForBrowserState(browser_state);
     if (profile_sync_service != nullptr) {
       const syncer::DeviceInfoTracker* tracker =
           profile_sync_service->GetDeviceInfoTracker();
       if (tracker != nullptr) {
         trackers->push_back(tracker);
       }
     }
   }
 }