Add missing IWYU message_loop.h includes.

content/shell/browser/shell.cc

 // Copyright 2013 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/shell.h"
 
 #include <stddef.h>
 
 #include "base/auto_reset.h"
 #include "base/command_line.h"
 #include "base/location.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 "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/navigation_controller.h"
@@ skipping 501 matching lines @@
   if (entry)
     PlatformSetTitle(entry->GetTitle());
 }
 
 void Shell::OnDevToolsWebContentsDestroyed() {
   devtools_observer_.reset();
   devtools_frontend_ = NULL;
 }
 
 }  // namespace content