Add missing IWYU message_loop.h includes.

media/cast/test/utility/udp_proxy.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 "media/cast/test/utility/udp_proxy.h"
 
 #include <math.h>
 #include <stdlib.h>
 #include <utility>
 #include <vector>
 
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/rand_util.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "base/time/default_tick_clock.h"
 #include "net/base/io_buffer.h"
 #include "net/base/net_errors.h"
 #include "net/log/net_log_source.h"
 #include "net/socket/udp_server_socket.h"
 
@@ skipping 830 matching lines @@
     net::NetLog* net_log) {
   std::unique_ptr<UDPProxy> ret(
       new UDPProxyImpl(local_port, destination, std::move(to_dest_pipe),
                        std::move(from_dest_pipe), net_log));
   return ret;
 }
 
 }  // namespace test
 }  // namespace cast
 }  // namespace media