[Sync] Move thread checking into the ModelSafeWorker interface.

ios/chrome/browser/sync/ios_chrome_profile_sync_service_factory.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 "ios/chrome/browser/sync/ios_chrome_profile_sync_service_factory.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "base/memory/singleton.h"
@@ skipping 123 matching lines @@
   init_params.oauth2_token_service =
       OAuth2TokenServiceFactory::GetForBrowserState(browser_state);
   init_params.start_behavior = ProfileSyncService::MANUAL_START;
   init_params.sync_client =
       base::MakeUnique<IOSChromeSyncClient>(browser_state);
   init_params.network_time_update_callback = base::Bind(&UpdateNetworkTime);
   init_params.base_directory = browser_state->GetStatePath();
   init_params.url_request_context = browser_state->GetRequestContext();
   init_params.debug_identifier = browser_state->GetDebugName();
   init_params.channel = ::GetChannel();
-  init_params.db_thread =
-      web::WebThread::GetTaskRunnerForThread(web::WebThread::DB);
-  init_params.file_thread =
-      web::WebThread::GetTaskRunnerForThread(web::WebThread::FILE);
   init_params.blocking_pool = web::WebThread::GetBlockingPool();
 
   auto pss = base::MakeUnique<ProfileSyncService>(std::move(init_params));
 
   // Will also initialize the sync client.
   pss->Initialize();
   return std::move(pss);
 }