Add missing IWYU message_loop.h includes.

ui/aura/window_event_dispatcher_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 "ui/aura/window_event_dispatcher.h"
 
 #include <stddef.h>
 
 #include <utility>
 #include <vector>
 
 #include "base/bind.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/test/histogram_tester.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "build/build_config.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "ui/aura/client/capture_client.h"
 #include "ui/aura/client/event_client.h"
 #include "ui/aura/client/focus_client.h"
 #include "ui/aura/env.h"
@@ skipping 2923 matching lines @@
   EXPECT_EQ(1, last_event_location_delegate.mouse_event_count());
   EXPECT_EQ(1, last_event_location_delegate.nested_message_loop_count());
   EXPECT_EQ(mouse_location, last_event_location_delegate.last_mouse_location());
 
   // After dispatch the location should fallback to that of the
   // WindowTreeClient, which defaults to 0,0.
   EXPECT_EQ(gfx::Point(0, 0), Env::GetInstance()->last_mouse_location());
 }
 
 }  // namespace aura