Add missing IWYU message_loop.h includes.

components/sync/driver/shared_change_processor_unittest.cc

 // Copyright 2014 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/driver/shared_change_processor.h"
 
 #include <cstddef>
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/compiler_specific.h"
 #include "base/memory/ptr_util.h"
+#include "base/message_loop/message_loop.h"
 #include "base/synchronization/waitable_event.h"
 #include "base/threading/thread.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/device_info/local_device_info_provider.h"
 #include "components/sync/driver/fake_sync_client.h"
 #include "components/sync/driver/generic_change_processor.h"
 #include "components/sync/driver/generic_change_processor_factory.h"
 #include "components/sync/driver/sync_api_component_factory.h"
 #include "components/sync/model/attachments/attachment_id.h"
 #include "components/sync/model/attachments/attachment_service.h"
@@ skipping 201 matching lines @@
 // creates AttachmentService and passes it back to the syncable service.
 TEST_F(SyncSharedChangeProcessorTest, ConnectWithAttachmentStore) {
   SetAttachmentStore();
   Connect();
   EXPECT_TRUE(HasAttachmentService());
 }
 
 }  // namespace
 
 }  // namespace syncer