[USS] Add SyncMetadataStore interface

components/sync/model_impl/passthrough_metadata_change_list.cc

 // Copyright 2016 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/model_impl/passthrough_metadata_change_list.h"
 
 #include "base/logging.h"
 #include "components/sync/protocol/entity_metadata.pb.h"
 #include "components/sync/protocol/model_type_state.pb.h"
 
@@ skipping 20 matching lines @@
     const std::string& storage_key,
     const sync_pb::EntityMetadata& metadata) {
   store_->WriteMetadata(batch_, storage_key, metadata.SerializeAsString());
 }
 
 void PassthroughMetadataChangeList::ClearMetadata(
     const std::string& storage_key) {
   store_->DeleteMetadata(batch_, storage_key);
 }
 
+base::Optional<ModelError> PassthroughMetadataChangeList::TakeError() {
+  NOTREACHED();
+  return {};
+}
+
 }  // namespace syncer