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

components/sync/model/fake_model_type_sync_bridge.h

Index: components/sync/model/fake_model_type_sync_bridge.h
diff --git a/components/sync/model/fake_model_type_sync_bridge.h b/components/sync/model/fake_model_type_sync_bridge.h
index 31d95aeb209e96962b1ecc48e8749fa26da6141c..854669bb632c4d8f70a7eda4e67b78c63e64032d 100644
--- a/components/sync/model/fake_model_type_sync_bridge.h
+++ b/components/sync/model/fake_model_type_sync_bridge.h
@@ -9,6 +9,7 @@
 #include <memory>
 #include <string>
 
+#include "base/optional.h"
 #include "components/sync/engine/non_blocking_sync_common.h"
 #include "components/sync/model/entity_data.h"
 #include "components/sync/model/metadata_batch.h"
@@ -103,10 +104,10 @@ class FakeModelTypeSyncBridge : public ModelTypeSyncBridge {
 
   // ModelTypeSyncBridge implementation
   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;