[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/sync_backend_host_impl.cc

 // 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.
 
 #include "components/sync/driver/glue/sync_backend_host_impl.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 55 matching lines @@
 }
 
 SyncBackendHostImpl::~SyncBackendHostImpl() {
   DCHECK(!core_.get() && !frontend_) << "Must call Shutdown before destructor.";
   DCHECK(!registrar_.get());
 }
 
 void SyncBackendHostImpl::Initialize(
     SyncFrontend* frontend,
     base::Thread* sync_thread,
-    const scoped_refptr<base::SingleThreadTaskRunner>& db_thread,
-    const scoped_refptr<base::SingleThreadTaskRunner>& file_thread,
     const WeakHandle<JsEventHandler>& event_handler,
     const GURL& sync_service_url,
     const std::string& sync_user_agent,
     const SyncCredentials& credentials,
     bool delete_sync_data_folder,
     bool enable_local_sync_backend,
     const base::FilePath& local_sync_backend_folder,
     std::unique_ptr<SyncManagerFactory> sync_manager_factory,
     const WeakHandle<UnrecoverableErrorHandler>& unrecoverable_error_handler,
     const base::Closure& report_unrecoverable_error_function,
     const HttpPostProviderFactoryGetter& http_post_provider_factory_getter,
     std::unique_ptr<SyncEncryptionHandler::NigoriState> saved_nigori_state) {
   CHECK(sync_thread);
   sync_thread_ = sync_thread;
 
-  registrar_ = base::MakeUnique<SyncBackendRegistrar>(
-      name_, sync_client_, ui_thread_, db_thread, file_thread);
+  registrar_ = base::MakeUnique<SyncBackendRegistrar>(name_, sync_client_);
 
   DCHECK(frontend);
   frontend_ = frontend;
 
   std::vector<scoped_refptr<ModelSafeWorker>> workers;
   registrar_->GetWorkers(&workers);
 
   EngineComponentsFactory::Switches factory_switches = {
       EngineComponentsFactory::ENCRYPTION_KEYSTORE,
       EngineComponentsFactory::BACKOFF_NORMAL};
@@ skipping 703 matching lines @@
     const SyncManager::ClearServerDataCallback& frontend_callback) {
   DCHECK(ui_thread_->BelongsToCurrentThread());
   frontend_callback.Run();
 }
 
 }  // namespace syncer
 
 #undef SDVLOG
 
 #undef SLOG