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

components/sync/model/stub_model_type_sync_bridge.h

Index: components/sync/model/stub_model_type_sync_bridge.h
diff --git a/components/sync/model/stub_model_type_sync_bridge.h b/components/sync/model/stub_model_type_sync_bridge.h
index 38d6f30ff3f9fa8d797cf3613dd483c73ea2c61c..1543dc63c5c8daff8bab86a1a643b1b1046f22d5 100644
--- a/components/sync/model/stub_model_type_sync_bridge.h
+++ b/components/sync/model/stub_model_type_sync_bridge.h
@@ -8,6 +8,7 @@
 #include <memory>
 #include <string>
 
+#include "base/optional.h"
 #include "components/sync/model/model_type_sync_bridge.h"
 
 namespace syncer {
@@ -22,10 +23,10 @@ class StubModelTypeSyncBridge : public ModelTypeSyncBridge {
   ~StubModelTypeSyncBridge() override;
 
   std::unique_ptr<MetadataChangeList> CreateMetadataChangeList() override;
-  ModelError MergeSyncData(
+  base::Optional<ModelError> MergeSyncData(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityDataMap entity_data_map) override;
-  ModelError ApplySyncChanges(
+  base::Optional<ModelError> ApplySyncChanges(
       std::unique_ptr<MetadataChangeList> metadata_change_list,
       EntityChangeList entity_changes) override;
   void GetData(StorageKeyList storage_keys, DataCallback callback) override;