[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/sync_backend_host.h

 // Copyright 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.
 
 #ifndef COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_
 #define COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 42 matching lines @@
 
   // Called on the frontend's thread to kick off asynchronous initialization.
   // Optionally deletes the "Sync Data" folder during init in order to make
   // sure we're starting fresh.
   //
   // |saved_nigori_state| is optional nigori state to restore from a previous
   // backend instance. May be null.
   virtual void 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& 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,
@@ skipping 130 matching lines @@
   // See SyncManager::OnCookieJarChanged.
   virtual void OnCookieJarChanged(bool account_mismatch, bool empty_jar) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncBackendHost);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_H_