Add missing IWYU message_loop.h includes.

chromeos/process_proxy/process_proxy_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 <gtest/gtest.h>
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 
 #include "base/bind.h"
 #include "base/location.h"
+#include "base/message_loop/message_loop.h"
 #include "base/process/kill.h"
 #include "base/process/process.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chromeos/process_proxy/process_proxy_registry.h"
 
 namespace chromeos {
 
@@ skipping 211 matching lines @@
   RunTest();
 }
 
 // Open new process, then kill it. Verifiy that we detect when the process dies.
 TEST_F(ProcessProxyTest, RegistryNotifiedOnProcessExit) {
   test_runner_.reset(new RegistryNotifiedOnProcessExitTestRunner());
   RunTest();
 }
 
 }  // namespace chromeos