Do not observe MessageLoop destruction from ModelSafeWorker.

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/callback.h"
-#include "base/compiler_specific.h"
 #include "base/macros.h"
+#include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
-#include "base/synchronization/condition_variable.h"
-#include "base/synchronization/lock.h"
-#include "components/sync/base/unrecoverable_error_info.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:
-  UIModelWorker(const scoped_refptr<base::SingleThreadTaskRunner>& ui_thread,
-                WorkerLoopDestructionObserver* observer);
+  explicit UIModelWorker(scoped_refptr<base::SingleThreadTaskRunner> ui_thread);
 
   // ModelSafeWorker implementation. Called on syncapi SyncerThread.
-  void RegisterForLoopDestruction() override;
   ModelSafeGroup GetModelSafeGroup() 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_