Use a direct include of the message_loop header in base/.

base/synchronization/waitable_event_watcher.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 BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 
 #include "base/base_export.h"
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
 #include "base/win/object_watcher.h"
 #else
 #include "base/callback.h"
-#include "base/message_loop.h"
+#include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #endif
 
 namespace base {
 
 class Flag;
 class AsyncWaiter;
 class AsyncCallbackTask;
 class WaitableEvent;
 
@@ skipping 79 matching lines @@
   scoped_refptr<WaitableEvent::WaitableEventKernel> kernel_;
 #endif
 
   WaitableEvent* event_;
   EventCallback callback_;
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_