[Sync] Move sync's ModelSafeWorker implementations to engine/.

components/sync/driver/glue/ui_model_worker.h

-// 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.
-
-#ifndef COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_
-#define COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_
-
-#include "base/macros.h"
-#include "base/memory/ref_counted.h"
-#include "base/single_thread_task_runner.h"
-#include "components/sync/engine/model_safe_worker.h"
-
-namespace syncer {
-
-// A ModelSafeWorker for UI models (e.g. bookmarks) that
-// accepts work requests from the syncapi that need to be fulfilled
-// from the MessageLoop home to the native model.
-class UIModelWorker : public ModelSafeWorker {
- public:
-  explicit UIModelWorker(scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
-
-  // ModelSafeWorker implementation. Called on syncapi SyncerThread.
-  ModelSafeGroup GetModelSafeGroup() override;
-  bool IsOnModelThread() override;
-
- protected:
-  SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
-
- private:
-  ~UIModelWorker() override;
-
-  // A reference to the UI thread's task runner.
-  const scoped_refptr<base::SingleThreadTaskRunner> ui_thread_;
-
-  DISALLOW_COPY_AND_ASSIGN(UIModelWorker);
-};
-
-}  // namespace syncer
-
-#endif  // COMPONENTS_SYNC_DRIVER_GLUE_UI_MODEL_WORKER_H_