Add missing IWYU message_loop.h includes.

extensions/test/extension_test_message_listener.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 "extensions/test/extension_test_message_listener.h"
 
+#include "base/message_loop/message_loop.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/string_util.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_source.h"
 #include "content/public/test/test_utils.h"
 #include "extensions/browser/api/test/test_api.h"
 #include "extensions/browser/notification_types.h"
 
 ExtensionTestMessageListener::ExtensionTestMessageListener(
     const std::string& expected_message,
@@ skipping 95 matching lines @@
     function_ = function;
     if (!will_reply_)
       Reply(std::string());
 
     if (waiting_) {
       waiting_ = false;
       base::MessageLoopForUI::current()->QuitWhenIdle();
     }
   }
 }