Add missing IWYU message_loop.h includes.

content/shell/browser/layout_test/layout_test_browser_main.cc

 // 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.
 
 #include "content/shell/browser/layout_test/layout_test_browser_main.h"
 
 #include <iostream>
 #include <memory>
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
 #include "base/logging.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/public/browser/browser_main_runner.h"
 #include "content/public/common/url_constants.h"
 #include "content/shell/browser/layout_test/blink_test_controller.h"
@@ skipping 124 matching lines @@
   exit_code = RunTests(main_runner);
   base::RunLoop().RunUntilIdle();
 
   content::Shell::CloseAllWindows();
 #if !defined(OS_ANDROID)
   main_runner->Shutdown();
 #endif
 
   return exit_code;
 }