Add missing IWYU message_loop.h includes.

content/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.
 
 // This file contains download browser tests that are known to be runnable
 // in a pure content context.  Over time tests should be migrated here.
 
 #include <stddef.h>
 #include <stdint.h>
 #include <utility>
 #include <vector>
 
 #include "base/callback_helpers.h"
 #include "base/feature_list.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/format_macros.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
+#include "base/message_loop/message_loop.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/run_loop.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/test/mock_entropy_provider.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_restrictions.h"
@@ skipping 2707 matching lines @@
 
   std::vector<DownloadItem*> downloads;
   DownloadManagerForShell(shell())->GetAllDownloads(&downloads);
   ASSERT_EQ(1u, downloads.size());
 
   EXPECT_EQ(FILE_PATH_LITERAL("foo"),
             downloads[0]->GetTargetFilePath().BaseName().value());
 }
 
 }  // namespace content