[USS] Add SyncMetadataStore interface

components/sync/model_impl/in_memory_metadata_change_list.cc

Index: components/sync/model_impl/in_memory_metadata_change_list.cc
diff --git a/components/sync/model_impl/in_memory_metadata_change_list.cc b/components/sync/model_impl/in_memory_metadata_change_list.cc
index 23c2a947fc11145a210134cbfbf91421fbd8fefe..e210874aa59dbb432d140b6e66a09713daf67cec 100644
--- a/components/sync/model_impl/in_memory_metadata_change_list.cc
+++ b/components/sync/model_impl/in_memory_metadata_change_list.cc
@@ -28,4 +28,9 @@ void InMemoryMetadataChangeList::ClearMetadata(const std::string& storage_key) {
   metadata_changes_[storage_key] = {CLEAR, sync_pb::EntityMetadata()};
 }
 
+base::Optional<ModelError> InMemoryMetadataChangeList::TakeError() {
+  NOTREACHED();

[pavely, 2017/04/06 22:56:42]

I don't think we should assert NOTREACHED() here. 
TakeError should either be restricted to only OnDiskMCL and therefore shouldn't
be present in MetadataChangeList and its derived classes, or it should be
allowed to call on any implementation of MetadataChangeList.

[Gang Wu, 2017/04/21 05:28:03]

Done.

+  return {};
+}
+
 }  // namespace syncer