Add missing IWYU message_loop.h includes.

chromeos/process_proxy/process_output_watcher_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 "chromeos/process_proxy/process_output_watcher.h"
 
 #include <gtest/gtest.h>
 #include <stddef.h>
 
 #include <memory>
 #include <queue>
 #include <string>
 #include <vector>
 
 #include "base/bind.h"
 #include "base/callback.h"
 #include "base/files/file_util.h"
 #include "base/location.h"
+#include "base/message_loop/message_loop.h"
 #include "base/posix/eintr_wrapper.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/strings/string_util.h"
 #include "base/threading/thread.h"
 
 namespace chromeos {
 
 namespace {
 
@@ skipping 292 matching lines @@
   // This will send '\0' to output watcher.
   test_cases.push_back(TestCase("", true));
   // Let's verify that next input also gets detected (i.e. output watcher does
   // not exit after seeing '\0' from previous test case).
   test_cases.push_back(TestCase("a", true));
 
   RunTest(test_cases);
 }
 
 }  // namespace chromeos