[Sync] Move thread checking into the ModelSafeWorker interface.

components/sync/engine/passive_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_ENGINE_PASSIVE_MODEL_WORKER_H_
 #define COMPONENTS_SYNC_ENGINE_PASSIVE_MODEL_WORKER_H_
 
 #include "base/macros.h"
 #include "components/sync/base/syncer_error.h"
 #include "components/sync/engine/model_safe_worker.h"
 
 namespace syncer {
 
 // Implementation of ModelSafeWorker for passive types.  All work is
 // done on the same thread DoWorkAndWaitUntilDone (i.e., the sync
 // thread).
 class PassiveModelWorker : public ModelSafeWorker {
  public:
   PassiveModelWorker();
 
-  // ModelSafeWorker implementation. Called on the sync thread.
+  // ModelSafeWorker implementation.
   ModelSafeGroup GetModelSafeGroup() override;
+  bool IsOnModelThread() override;
 
  protected:
   SyncerError DoWorkAndWaitUntilDoneImpl(const WorkCallback& work) override;
 
  private:
   ~PassiveModelWorker() override;
 
   DISALLOW_COPY_AND_ASSIGN(PassiveModelWorker);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_ENGINE_PASSIVE_MODEL_WORKER_H_