[USS] Add SyncMetadataStore interface

components/sync/model_impl/passthrough_metadata_change_list.h

 // 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.
 
 #ifndef COMPONENTS_SYNC_MODEL_IMPL_PASSTHROUGH_METADATA_CHANGE_LIST_H_
 #define COMPONENTS_SYNC_MODEL_IMPL_PASSTHROUGH_METADATA_CHANGE_LIST_H_
 
 #include <string>
 
 #include "components/sync/model/metadata_change_list.h"
 #include "components/sync/model/model_type_store.h"
 
 namespace syncer {
 
 // A thin wrapper around ModelTypeStore and WriteBatch to implement the
 // MetadataChangeList interface. This class should never outlive any of the
 // delegate objects it calls.
 class PassthroughMetadataChangeList : public MetadataChangeList {
  public:
   PassthroughMetadataChangeList(ModelTypeStore* store,
                                 ModelTypeStore::WriteBatch* batch);
 
   // DataBatch MetadataChangeList.
   void UpdateModelTypeState(
       const sync_pb::ModelTypeState& model_type_state) override;
   void ClearModelTypeState() override;
   void UpdateMetadata(const std::string& storage_key,
                       const sync_pb::EntityMetadata& metadata) override;
   void ClearMetadata(const std::string& storage_key) override;
+  base::Optional<syncer::ModelError> TakeError() override;
 
  private:
   // Non owning references.
   ModelTypeStore* store_;
   ModelTypeStore::WriteBatch* batch_;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_IMPL_PASSTHROUGH_METADATA_CHANGE_LIST_H_