Add missing IWYU message_loop.h includes.

components/sync/engine/sync_backend_registrar_unittest.cc

 // Copyright 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/sync_backend_registrar.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/single_thread_task_runner.h"
 #include "base/threading/thread.h"
 #include "components/sync/engine/browser_thread_model_worker.h"
 #include "components/sync/engine/passive_model_worker.h"
 #include "components/sync/model/change_processor_mock.h"
 #include "components/sync/syncable/test_user_share.h"
 #include "testing/gmock/include/gmock/gmock.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
@@ skipping 244 matching lines @@
       registrar()->ConfigureDataTypes(types_to_add, ModelTypeSet());
   EXPECT_EQ(ModelTypeSet(BOOKMARKS), newly_added_types);
   EXPECT_EQ(types_to_add, registrar()->GetLastConfiguredTypes());
   ExpectRoutingInfo(
       {{AUTOFILL, GROUP_NON_BLOCKING}, {BOOKMARKS, GROUP_NON_BLOCKING}});
 }
 
 }  // namespace
 
 }  // namespace syncer