Add missing IWYU message_loop.h includes.

components/browser_watcher/window_hang_monitor_win_unittest.cc

 // Copyright 2015 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 "components/browser_watcher/window_hang_monitor_win.h"
 
 #include <memory>
 
 #include "base/base_paths.h"
 #include "base/base_switches.h"
 #include "base/command_line.h"
+#include "base/message_loop/message_loop.h"
 #include "base/path_service.h"
 #include "base/process/launch.h"
 #include "base/process/process.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/test/multiprocess_test.h"
 #include "base/threading/thread.h"
 #include "base/win/message_window.h"
 #include "base/win/scoped_handle.h"
 #include "base/win/win_util.h"
@@ skipping 446 matching lines @@
 
   ASSERT_TRUE(SendSignal(IPC_SIGNAL_HANG_MESSAGE_WINDOW));
 
   EXPECT_EQ(WindowHangMonitor::WINDOW_HUNG,
             monitor_thread().WaitForEvent());
 
   ASSERT_TRUE(SendSignal(IPC_SIGNAL_TERMINATE_PROCESS));
 }
 
 }  // namespace browser_watcher