Add missing IWYU message_loop.h includes.

chrome/browser/chromeos/file_manager/file_manager_browsertest_base.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 "chrome/browser/chromeos/file_manager/file_manager_browsertest_base.h"
 
 #include <stddef.h>
 
 #include <deque>
 
 #include "base/json/json_reader.h"
 #include "base/json/json_value_converter.h"
 #include "base/json/json_writer.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/run_loop.h"
 #include "base/strings/string_piece.h"
 #include "base/threading/thread_restrictions.h"
 #include "base/time/time.h"
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/chromeos/drive/file_system_util.h"
 #include "chrome/browser/chromeos/file_manager/app_id.h"
 #include "chrome/browser/chromeos/file_manager/mount_test_util.h"
 #include "chrome/browser/chromeos/file_manager/path_util.h"
@@ skipping 729 matching lines @@
 }
 
 drive::DriveIntegrationService*
 FileManagerBrowserTestBase::CreateDriveIntegrationService(Profile* profile) {
   drive_volumes_[profile->GetOriginalProfile()].reset(new DriveTestVolume());
   return drive_volumes_[profile->GetOriginalProfile()]
       ->CreateDriveIntegrationService(profile);
 }
 
 }  // namespace file_manager