Add missing IWYU message_loop.h includes.

ui/views/mus/views_mus_test_suite.cc

 // Copyright 2016 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 "ui/views/mus/views_mus_test_suite.h"
 
 #include <memory>
 #include <string>
 
 #include "base/command_line.h"
 #include "base/files/file_path.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/simple_thread.h"
 #include "base/threading/thread.h"
 #include "mojo/edk/embedder/embedder.h"
 #include "mojo/edk/embedder/scoped_ipc_support.h"
 #include "services/catalog/catalog.h"
 #include "services/service_manager/background/background_service_manager.h"
 #include "services/service_manager/public/cpp/connector.h"
 #include "services/service_manager/public/cpp/service.h"
@@ skipping 240 matching lines @@
 
 void ViewsMusTestSuite::InitializeEnv() {
   env_ = aura::Env::CreateInstance(aura::Env::Mode::MUS);
 }
 
 void ViewsMusTestSuite::DestroyEnv() {
   env_.reset();
 }
 
 }  // namespace views