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

components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc

Index: components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
diff --git a/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc b/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
index 304cfe1d09f19e282e97e1d43870393f2fc9728d..5b649892599217e67fadfbfac4a04e9729ecfce9 100644
--- a/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
+++ b/components/autofill/core/browser/webdata/autocomplete_sync_bridge.cc
@@ -104,20 +104,20 @@ AutocompleteSyncBridge::CreateMetadataChangeList() {
                                                       syncer::AUTOFILL);
 }
 
-syncer::ModelError AutocompleteSyncBridge::MergeSyncData(
+base::Optional<syncer::ModelError> AutocompleteSyncBridge::MergeSyncData(
     std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
     syncer::EntityDataMap entity_data_map) {
   DCHECK(thread_checker_.CalledOnValidThread());
   NOTIMPLEMENTED();
-  return syncer::ModelError();
+  return {};
 }
 
-syncer::ModelError AutocompleteSyncBridge::ApplySyncChanges(
+base::Optional<syncer::ModelError> AutocompleteSyncBridge::ApplySyncChanges(
     std::unique_ptr<syncer::MetadataChangeList> metadata_change_list,
     syncer::EntityChangeList entity_changes) {
   DCHECK(thread_checker_.CalledOnValidThread());
   NOTIMPLEMENTED();
-  return syncer::ModelError();
+  return {};
 }
 
 void AutocompleteSyncBridge::AutocompleteSyncBridge::GetData(