Add missing IWYU message_loop.h includes.

chrome/browser/ui/webui/md_downloads/md_downloads_dom_handler.cc

 // Copyright (c) 2012 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/ui/webui/md_downloads/md_downloads_dom_handler.h"
 
 #include <algorithm>
 #include <functional>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/i18n/rtl.h"
 #include "base/logging.h"
+#include "base/message_loop/message_loop.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_piece.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/supports_user_data.h"
 #include "base/threading/thread.h"
 #include "base/values.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/download/download_danger_prompt.h"
 #include "chrome/browser/download/download_history.h"
@@ skipping 428 matching lines @@
 
 void MdDownloadsDOMHandler::RemoveDownloadInArgs(const base::ListValue* args) {
   content::DownloadItem* file = GetDownloadByValue(args);
   if (!file)
     return;
 
   DownloadVector downloads;
   downloads.push_back(file);
   RemoveDownloads(downloads);
 }