Add missing IWYU message_loop.h includes.

chrome/browser/download/download_browsertest.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/download/download_browsertest.h"
 
 #include <stdint.h>
 
 #include <memory>
 #include <set>
 #include <sstream>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/command_line.h"
 #include "base/feature_list.h"
 #include "base/files/file.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_split.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/sys_info.h"
 #include "base/test/test_file_util.h"
 #include "base/threading/thread_restrictions.h"
 #include "build/build_config.h"
@@ skipping 3831 matching lines @@
           browser(), 1,
           content::DownloadTestObserver::ON_DANGEROUS_DOWNLOAD_DENY));
   ui_test_utils::NavigateToURL(browser(), extension_url);
 
   observer->WaitForFinished();
 
   // Download shelf should close.
   EXPECT_FALSE(browser()->window()->IsDownloadShelfVisible());
 }
 #endif  // defined(OS_CHROMEOS)