Add missing IWYU message_loop.h includes.

ipc/ipc_sync_channel_unittest.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 "ipc/ipc_sync_channel.h"
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/location.h"
 #include "base/logging.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/process/process_handle.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_util.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "ipc/ipc_listener.h"
@@ skipping 1836 matching lines @@
   workers.push_back(new ReentrantReplyServer2(std::move(pipe2.handle0)));
   workers.push_back(new ReentrantReplyServer1(
       &server_ready, std::move(pipe1.handle0), std::move(pipe2.handle1)));
   workers.push_back(
       new ReentrantReplyClient(&server_ready, std::move(pipe1.handle1)));
   RunTest(workers);
 }
 
 }  // namespace
 }  // namespace IPC