[Sync] Remove ModelError::IsSet() in favor of base::Optional.

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 747303c91f6bcfda1cafee690ba4dca7a1f945cc..c5bbfe021e58b15bf967da5383e437d06c2b6541 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>();
 }
 
-ModelError StubModelTypeSyncBridge::MergeSyncData(
+base::Optional<ModelError> StubModelTypeSyncBridge::MergeSyncData(
     std::unique_ptr<MetadataChangeList> metadata_change_list,
     EntityDataMap entity_data_map) {
-  return ModelError();
+  return {};
 }
 
-ModelError StubModelTypeSyncBridge::ApplySyncChanges(
+base::Optional<ModelError> StubModelTypeSyncBridge::ApplySyncChanges(
     std::unique_ptr<MetadataChangeList> metadata_change_list,
     EntityChangeList entity_changes) {
-  return ModelError();
+  return {};
 }
 
 void StubModelTypeSyncBridge::GetData(StorageKeyList storage_keys,