[USS] Add SyncMetadataStore interface

components/sync/model_impl/in_memory_metadata_change_list.h

Index: components/sync/model_impl/in_memory_metadata_change_list.h
diff --git a/components/sync/model_impl/in_memory_metadata_change_list.h b/components/sync/model_impl/in_memory_metadata_change_list.h
index f077087db83c3f7ee44589da8a84f0778197394d..7d482fe687e15172651cb6bb474061241d4c17b6 100644
--- a/components/sync/model_impl/in_memory_metadata_change_list.h
+++ b/components/sync/model_impl/in_memory_metadata_change_list.h
@@ -41,6 +41,7 @@ class InMemoryMetadataChangeList : public MetadataChangeList {
   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;
 
  protected:
   // Protected because this class is useless to instantiate directly.