Add missing IWYU message_loop.h includes.

content/public/test/test_utils.cc

 // 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.
 
 #include "content/public/test/test_utils.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task_scheduler/task_scheduler.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/values.h"
 #include "build/build_config.h"
 #include "content/common/site_isolation_policy.h"
 #include "content/common/url_schemes.h"
@@ skipping 377 matching lines @@
 
 void WebContentsDestroyedWatcher::Wait() {
   run_loop_.Run();
 }
 
 void WebContentsDestroyedWatcher::WebContentsDestroyed() {
   run_loop_.Quit();
 }
 
 }  // namespace content