Add missing IWYU message_loop.h includes.

headless/lib/browser/headless_browser_impl.cc

 // Copyright 2015 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 "headless/lib/browser/headless_browser_impl.h"
 
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/command_line.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "content/public/app/content_main.h"
 #include "content/public/browser/browser_thread.h"
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/web_contents.h"
 #include "content/public/common/content_switches.h"
 #include "headless/app/headless_shell_switches.h"
 #include "headless/lib/browser/headless_browser_context_impl.h"
 #include "headless/lib/browser/headless_browser_main_parts.h"
 #include "headless/lib/browser/headless_devtools_client_impl.h"
@@ skipping 245 matching lines @@
 
   // Child processes should not end up here.
   DCHECK(!base::CommandLine::ForCurrentProcess()->HasSwitch(
       ::switches::kProcessType));
 #endif
   return RunContentMain(std::move(options),
                         std::move(on_browser_start_callback));
 }
 
 }  // namespace headless