[Sync] Move thread checking into the ModelSafeWorker interface.

chrome/browser/sync/profile_sync_service_factory.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/sync/profile_sync_service_factory.h"
 
 #include <utility>
 
 #include "base/memory/ptr_util.h"
 #include "base/memory/singleton.h"
@@ skipping 165 matching lines @@
         base::MakeUnique<browser_sync::ChromeSyncClient>(profile);
   } else {
     init_params.sync_client = client_factory_->Run(profile);
   }
 
   init_params.network_time_update_callback = base::Bind(&UpdateNetworkTime);
   init_params.base_directory = profile->GetPath();
   init_params.url_request_context = profile->GetRequestContext();
   init_params.debug_identifier = profile->GetDebugName();
   init_params.channel = chrome::GetChannel();
-
-  init_params.db_thread = content::BrowserThread::GetTaskRunnerForThread(
-      content::BrowserThread::DB);
-  init_params.file_thread = content::BrowserThread::GetTaskRunnerForThread(
-      content::BrowserThread::FILE);
   init_params.blocking_pool = content::BrowserThread::GetBlockingPool();
 
   auto pss = base::MakeUnique<ProfileSyncService>(std::move(init_params));
 
   // Will also initialize the sync client.
   pss->Initialize();
   return pss.release();
 }
 
 // static
 bool ProfileSyncServiceFactory::HasProfileSyncService(Profile* profile) {
   return GetInstance()->GetServiceForBrowserContext(profile, false) != nullptr;
 }
 
 // static
 void ProfileSyncServiceFactory::SetSyncClientFactoryForTest(
     SyncClientFactory* client_factory) {
   client_factory_ = client_factory;
 }
 
 // static
 ProfileSyncServiceFactory::SyncClientFactory*
     ProfileSyncServiceFactory::client_factory_ = nullptr;