Add missing IWYU message_loop.h includes.

third_party/WebKit/public/platform/scheduler/child/webthread_base.h

 // Copyright 2014 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 THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_CHILD_WEBTHREAD_BASE_H_
 #define THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_CHILD_WEBTHREAD_BASE_H_
 
 #include <map>
 #include <memory>
 
+#include "base/message_loop/message_loop.h"
 #include "base/threading/thread.h"
 #include "public/platform/WebCommon.h"
 #include "public/platform/WebThread.h"
 #include "public/platform/WebTraceLocation.h"
 
 namespace blink {
 namespace scheduler {
 class SingleThreadIdleTaskRunner;
 class TaskTimeObserver;
 
@@ skipping 52 matching lines @@
 
  private:
   typedef std::map<TaskObserver*, TaskObserverAdapter*> TaskObserverMap;
   TaskObserverMap task_observer_map_;
 };
 
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_PUBLIC_PLATFORM_SCHEDULER_CHILD_WEBTHREAD_BASE_H_