[USS] Add SyncMetadataStore interface

components/sync/model_impl/in_memory_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_IN_MEMORY_METADATA_CHANGE_LIST_H_
 #define COMPONENTS_SYNC_MODEL_IMPL_IN_MEMORY_METADATA_CHANGE_LIST_H_
 
 #include <map>
 #include <memory>
 #include <string>
@@ skipping 23 matching lines @@
 
   ~InMemoryMetadataChangeList() override;
 
   // MetadataChangeList implementation.
   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;
 
  protected:
   // Protected because this class is useless to instantiate directly.
   InMemoryMetadataChangeList();
 
   std::map<std::string, MetadataChange> metadata_changes_;
   std::unique_ptr<ModelTypeStateChange> state_change_;
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_MODEL_IMPL_IN_MEMORY_METADATA_CHANGE_LIST_H_