[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/driver/glue/browser_thread_model_worker.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_BROWSER_THREAD_MODEL_WORKER_H_
 #define COMPONENTS_SYNC_DRIVER_GLUE_BROWSER_THREAD_MODEL_WORKER_H_
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/single_thread_task_runner.h"
 #include "components/sync/base/scoped_event_signal.h"
 #include "components/sync/base/syncer_error.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
 
 // A ModelSafeWorker for models that accept requests from the
 // syncapi that need to be fulfilled on a browser thread, for example
 // autofill on the DB thread.
 // TODO(sync): Try to generalize other ModelWorkers (e.g. history, etc).
 class BrowserThreadModelWorker : public ModelSafeWorker {
  public:
   BrowserThreadModelWorker(
       const scoped_refptr<base::SingleThreadTaskRunner>& runner,
       ModelSafeGroup group);
 
-  // ModelSafeWorker implementation. Called on the sync thread.
+  // ModelSafeWorker implementation.
   ModelSafeGroup GetModelSafeGroup() override;
+  bool IsOnModelThread() override;
 
  protected:
   ~BrowserThreadModelWorker() override;
 
   SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
 
   void CallDoWorkAndSignalTask(const WorkCallback& work,
                                syncer::ScopedEventSignal scoped_event_signal,
                                SyncerError* error);
 
  private:
   scoped_refptr<base::SingleThreadTaskRunner> runner_;
   ModelSafeGroup group_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserThreadModelWorker);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_GLUE_BROWSER_THREAD_MODEL_WORKER_H_