Add missing IWYU message_loop.h includes.

ipc/ipc_channel_proxy_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 "build/build_config.h"
 
 #include <stddef.h>
 #include <stdint.h>
 #include <memory>
 
+#include "base/message_loop/message_loop.h"
 #include "base/pickle.h"
 #include "base/run_loop.h"
 #include "base/threading/thread.h"
 #include "ipc/ipc_message.h"
 #include "ipc/ipc_test_base.h"
 #include "ipc/message_filter.h"
 
 // Get basic type definitions.
 #define IPC_MESSAGE_IMPL
 #include "ipc/ipc_channel_proxy_unittest_messages.h"
@@ skipping 404 matching lines @@
 DEFINE_IPC_CHANNEL_MOJO_TEST_CLIENT(ChannelProxyClient) {
   ChannelReflectorListener listener;
   Connect(&listener);
   listener.Init(channel());
 
   base::RunLoop().Run();
   Close();
 }
 
 }  // namespace