[Sync] Introduce ModelError for USS error handling.

components/sync/model/stub_model_type_sync_bridge.cc

Index: components/sync/model/stub_model_type_sync_bridge.cc
diff --git a/components/sync/model/stub_model_type_sync_bridge.cc b/components/sync/model/stub_model_type_sync_bridge.cc
index b07eff4d13f8f89b6427774a65d98dcee8877367..747303c91f6bcfda1cafee690ba4dca7a1f945cc 100644
--- a/components/sync/model/stub_model_type_sync_bridge.cc
+++ b/components/sync/model/stub_model_type_sync_bridge.cc
@@ -25,16 +25,16 @@ StubModelTypeSyncBridge::CreateMetadataChangeList() {
   return std::unique_ptr<MetadataChangeList>();
 }
 
-SyncError StubModelTypeSyncBridge::MergeSyncData(
+ModelError StubModelTypeSyncBridge::MergeSyncData(
     std::unique_ptr<MetadataChangeList> metadata_change_list,
     EntityDataMap entity_data_map) {
-  return SyncError();
+  return ModelError();
 }
 
-SyncError StubModelTypeSyncBridge::ApplySyncChanges(
+ModelError StubModelTypeSyncBridge::ApplySyncChanges(
     std::unique_ptr<MetadataChangeList> metadata_change_list,
     EntityChangeList entity_changes) {
-  return SyncError();
+  return ModelError();
 }
 
 void StubModelTypeSyncBridge::GetData(StorageKeyList storage_keys,