Add missing IWYU message_loop.h includes.

chrome/browser/extensions/extension_user_script_loader_unittest.cc

 // Copyright 2014 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 "extensions/browser/extension_user_script_loader.h"
 
 #include <stddef.h>
 
 #include <set>
 #include <string>
 
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_util.h"
 #include "chrome/browser/chrome_notification_types.h"
 #include "chrome/test/base/testing_profile.h"
 #include "content/public/browser/notification_observer.h"
 #include "content/public/browser/notification_registrar.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/test/test_browser_thread.h"
 #include "extensions/common/host_id.h"
@@ skipping 245 matching lines @@
       &profile,
       HostID(),
       true /* listen_for_extension_system_loaded */);
   loader.LoadScriptsForTest(&user_scripts);
 
   EXPECT_EQ(content,
             user_scripts[0]->js_scripts()[0]->GetContent().as_string());
 }
 
 }  // namespace extensions