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

components/sync/model_impl/shared_model_type_processor_unittest.cc

Index: components/sync/model_impl/shared_model_type_processor_unittest.cc
diff --git a/components/sync/model_impl/shared_model_type_processor_unittest.cc b/components/sync/model_impl/shared_model_type_processor_unittest.cc
index 74da18360e3b7336714094a2911148c3bdaf9ca2..73dc7b1fef64fd96276779b3703b35edbaec10e7 100644
--- a/components/sync/model_impl/shared_model_type_processor_unittest.cc
+++ b/components/sync/model_impl/shared_model_type_processor_unittest.cc
@@ -92,8 +92,9 @@ class TestModelTypeSyncBridge : public FakeModelTypeSyncBridge {
 
   // FakeModelTypeSyncBridge overrides.
 
-  ModelError MergeSyncData(std::unique_ptr<MetadataChangeList> mcl,
-                           EntityDataMap entity_data_map) override {
+  base::Optional<ModelError> MergeSyncData(
+      std::unique_ptr<MetadataChangeList> mcl,
+      EntityDataMap entity_data_map) override {
     merge_call_count_++;
     return FakeModelTypeSyncBridge::MergeSyncData(std::move(mcl),
                                                   entity_data_map);