Add missing IWYU message_loop.h includes.

components/sync/engine/ui_model_worker_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 "components/sync/engine/ui_model_worker.h"
 
 #include <memory>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/run_loop.h"
 #include "base/test/test_timeouts.h"
 #include "base/threading/platform_thread.h"
 #include "base/threading/thread.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace syncer {
 namespace {
 
@@ skipping 87 matching lines @@
   // RequestStop() is called. However, it doesn't fail (no flakes).
   base::PlatformThread::Sleep(TestTimeouts::tiny_timeout());
 
   // The sync thread shouldn't wait for the WorkCallback to run on the UI thread
   // after RequestStop() is called.
   worker_->RequestStop();
   sync_thread_.Stop();
 }
 
 }  // namespace syncer