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

components/sync/model/model_type_store.h

Index: components/sync/model/model_type_store.h
diff --git a/components/sync/model/model_type_store.h b/components/sync/model/model_type_store.h
index dd361f390e8ae4b5d060702dc28dad5abf516b7d..d640c9eb5d78ac6032ff05173b33a0524748aa82 100644
--- a/components/sync/model/model_type_store.h
+++ b/components/sync/model/model_type_store.h
@@ -11,6 +11,7 @@
 
 #include "base/callback.h"
 #include "base/macros.h"
+#include "base/optional.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/model/metadata_batch.h"
 #include "components/sync/model/metadata_change_list.h"
@@ -119,7 +120,7 @@ class ModelTypeStore {
   typedef base::Callback<void(Result result,
                               std::unique_ptr<RecordList> data_records)>
       ReadAllDataCallback;
-  typedef base::Callback<void(ModelError error,
+  typedef base::Callback<void(base::Optional<ModelError> error,
                               std::unique_ptr<MetadataBatch> metadata_batch)>
       ReadMetadataCallback;