Convert WaitableEvent::EventCallback to OnceCallback

base/synchronization/waitable_event_watcher_win.cc

 // 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.
 
 #include "base/synchronization/waitable_event_watcher.h"
 
 #include "base/compiler_specific.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/win/object_watcher.h"
 
 namespace base {
 
 WaitableEventWatcher::WaitableEventWatcher() = default;
 
 WaitableEventWatcher::~WaitableEventWatcher() {
 }
 
-bool WaitableEventWatcher::StartWatching(
-    WaitableEvent* event,
-    const EventCallback& callback) {
-  callback_ = callback;
+bool WaitableEventWatcher::StartWatching(WaitableEvent* event,
+                                         EventCallback callback) {
+  callback_ = std::move(callback);
   event_ = event;
   return watcher_.StartWatchingOnce(event->handle(), this);
 }
 
 void WaitableEventWatcher::StopWatching() {
   callback_.Reset();
   event_ = NULL;
   watcher_.StopWatching();
 }
 
 void WaitableEventWatcher::OnObjectSignaled(HANDLE h) {
   WaitableEvent* event = event_;
-  EventCallback callback = callback_;
+  EventCallback callback = std::move(callback_);
   event_ = NULL;
-  callback_.Reset();
   DCHECK(event);
 
-  callback.Run(event);
+  std::move(callback).Run(event);
 }
 
 }  // namespace base