[Sync] Refactor ModelSafeWorker::DoWorkAndWaitUntilDone() to avoid code duplication.

components/sync/base/scoped_event_signal.h

-// Copyright 2016 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_BASE_SCOPED_EVENT_SIGNAL_H_
-#define COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_
-
-#include "base/macros.h"
-
-namespace base {
-class WaitableEvent;
-}
-
-namespace syncer {
-
-// An object which signals a WaitableEvent when it is deleted. Used to wait for
-// a task to run or be abandoned.
-class ScopedEventSignal {
- public:
-  // |event| will be signaled in the destructor.
-  explicit ScopedEventSignal(base::WaitableEvent* event);
-
-  // This ScopedEventSignal will signal |other|'s WaitableEvent in its
-  // destructor. |other| will not signal anything in its destructor. The
-  // assignment operator cannot be used if this ScopedEventSignal's
-  // WaitableEvent hasn't been moved to another ScopedEventSignal.
-  ScopedEventSignal(ScopedEventSignal&& other);
-  ScopedEventSignal& operator=(ScopedEventSignal&& other);
-
-  ~ScopedEventSignal();
-
- private:
-  base::WaitableEvent* event_;
-
-  DISALLOW_COPY_AND_ASSIGN(ScopedEventSignal);
-};
-
-}  // namespace syncer
-
-#endif  // COMPONENTS_SYNC_BASE_SCOPED_EVENT_SIGNAL_H_