[Sync] Pass a TaskRunner into SBHI/SBHC, not a thread or message loop.

components/sync/driver/glue/sync_backend_host_impl.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_IMPL_H_
 #define COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_IMPL_H_
 
 #include <stdint.h>
 
 #include <map>
@@ skipping 53 matching lines @@
       SyncClient* sync_client,
       const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread,
       invalidation::InvalidationService* invalidator,
       const base::WeakPtr<SyncPrefs>& sync_prefs,
       const base::FilePath& sync_folder);
   ~SyncBackendHostImpl() override;
 
   // SyncBackendHost implementation.
   void Initialize(
       SyncFrontend* frontend,
-      base::Thread* sync_thread,
+      scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner,
       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 202 matching lines @@
 
   void ClearServerDataDoneOnFrontendLoop(
       const SyncManager::ClearServerDataCallback& frontend_callback);
 
   // A reference to the TaskRUnner used to construct |this|, so we know how to
   // safely talk back to the SyncFrontend.
   scoped_refptr<base::SingleThreadTaskRunner> const frontend_task_runner_;
 
   SyncClient* const sync_client_;
 
-  // A pointer to the sync thread.
-  base::Thread* sync_thread_;
+  // The task runner where all the sync engine operations happen.
+  scoped_refptr<base::SingleThreadTaskRunner> sync_task_runner_;
 
   // The UI thread's task runner.
   const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
 
   // Name used for debugging (set from profile_->GetDebugName()).
   const std::string name_;
 
   // Our core, which communicates directly to the syncapi. Use refptr instead
   // of WeakHandle because |core_| is created on UI loop but released on
   // sync loop.
@@ skipping 39 matching lines @@
   bool invalidation_handler_registered_;
 
   base::WeakPtrFactory<SyncBackendHostImpl> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncBackendHostImpl);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_GLUE_SYNC_BACKEND_HOST_IMPL_H_