[Sync] Make ModelSafeWorker a true interface

chrome/browser/sync/engine/passive_model_worker.h

+// Copyright (c) 2011 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 CHROME_BROWSER_SYNC_ENGINE_PASSIVE_MODEL_WORKER_H_
+#define CHROME_BROWSER_SYNC_ENGINE_PASSIVE_MODEL_WORKER_H_
+#pragma once
+
+#include "base/basictypes.h"
+#include "base/compiler_specific.h"
+#include "chrome/browser/sync/engine/model_safe_worker.h"
+
+class MessageLoop;
+
+namespace browser_sync {
+
+// 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:
+  explicit PassiveModelWorker(const MessageLoop* sync_loop);
+
+  // ModelSafeWorker implementation. Called on the sync thread.
+  virtual UnrecoverableErrorInfo DoWorkAndWaitUntilDone(
+      const WorkCallback& work) OVERRIDE;
+  virtual ModelSafeGroup GetModelSafeGroup() OVERRIDE;
+
+ private:
+  virtual ~PassiveModelWorker();
+
+  const MessageLoop* const sync_loop_;
+
+  DISALLOW_COPY_AND_ASSIGN(PassiveModelWorker);
+};
+
+}  // namespace browser_sync
+
+#endif  // CHROME_BROWSER_SYNC_ENGINE_PASSIVE_MODEL_WORKER_H_