[Sync] Introduce ModelError for USS error handling.

components/sync/model_impl/model_type_store_impl.h

Index: components/sync/model_impl/model_type_store_impl.h
diff --git a/components/sync/model_impl/model_type_store_impl.h b/components/sync/model_impl/model_type_store_impl.h
index f9b67336981790ef2a50238c84bbd9d648e7a746..343e496698e3640e6d91e3907b500079eb1c4015 100644
--- a/components/sync/model_impl/model_type_store_impl.h
+++ b/components/sync/model_impl/model_type_store_impl.h
@@ -111,12 +111,6 @@ class ModelTypeStoreImpl : public ModelTypeStore, public base::NonThreadSafe {
                            const std::string& global_metadata,
                            std::unique_ptr<RecordList> metadata_records);
 
-  // Helper function to create a SyncError with message |msg|.
-  SyncError MakeSyncError(const std::string& msg);
-
-  // The model type using this store.
-  const ModelType type_;
-
   // Backend should be deleted on backend thread.
   // To accomplish this store's dtor posts task to backend thread passing
   // backend ownership to task parameter.