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

components/sync/base/scoped_event_signal.cc

-// 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.
-
-#include "components/sync/base/scoped_event_signal.h"
-
-#include "base/logging.h"
-#include "base/synchronization/waitable_event.h"
-
-namespace syncer {
-
-ScopedEventSignal::ScopedEventSignal(base::WaitableEvent* event)
-    : event_(event) {
-  DCHECK(event_);
-}
-
-ScopedEventSignal::ScopedEventSignal(ScopedEventSignal&& other)
-    : event_(other.event_) {
-  other.event_ = nullptr;
-}
-
-ScopedEventSignal& ScopedEventSignal::operator=(ScopedEventSignal&& other) {
-  DCHECK(!event_);
-  event_ = other.event_;
-  other.event_ = nullptr;
-  return *this;
-}
-
-ScopedEventSignal::~ScopedEventSignal() {
-  if (event_)
-    event_->Signal();
-}
-
-}  // namespace syncer