Add missing IWYU message_loop.h includes.

base/files/file_path_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 "base/files/file_path_watcher.h"
 
 #if defined(OS_WIN)
 #include <windows.h>
 #include <aclapi.h>
 #elif defined(OS_POSIX)
 #include <sys/stat.h>
 #endif
 
 #include <set>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/location.h"
 #include "base/macros.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/stl_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/test/test_file_util.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
@@ skipping 829 matching lines @@
   // longer access the file
   ASSERT_TRUE(ChangeFilePermissions(test_dir1, Execute, false));
   ASSERT_TRUE(WaitForEvents());
   ASSERT_TRUE(ChangeFilePermissions(test_dir1, Execute, true));
 }
 
 #endif  // OS_MACOSX
 }  // namespace
 
 }  // namespace base