Add missing IWYU message_loop.h includes.

chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_test_utils.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 "chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager_test_utils.h"
 
 #include <stddef.h>
 
 #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/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "chrome/browser/chromeos/login/users/wallpaper/wallpaper_manager.h"
 #include "chromeos/chromeos_switches.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/gfx/codec/jpeg_codec.h"
 #include "ui/gfx/geometry/point.h"
 #include "ui/gfx/geometry/rect.h"
 
@@ skipping 199 matching lines @@
                             kWallpaperSize,
                             kLargeChildWallpaperColor));
 
   command_line->reset(new base::CommandLine(options));
   WallpaperManager::Get()->SetCommandLineForTesting(command_line->get());
 }
 
 }  // namespace wallpaper_manager_test_utils
 
 }  // namespace chromeos