Add missing IWYU message_loop.h includes.

ui/app_list/demo/app_list_demo_views.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 <stddef.h>
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/command_line.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 "build/build_config.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/common/content_switches.h"
 #include "ui/app_list/test/app_list_test_model.h"
 #include "ui/app_list/test/app_list_test_view_delegate.h"
 #include "ui/app_list/views/app_list_view.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/resources/grit/ui_resources.h"
@@ skipping 89 matching lines @@
 }
 
 }  // namespace
 
 int main(int argc, const char** argv) {
   ui::ViewsContentClient views_content_client(argc, argv);
 
   views_content_client.set_task(base::Bind(&ShowAppList));
   return views_content_client.RunMain();
 }