It seems this API is not used since it was introduced here: https://trac.webkit.org/changeset/210835/webkit
Created attachment 346151 [details] Patch
Comment on attachment 346151 [details] Patch Clearing flags on attachment: 346151 Committed r234422: <https://trac.webkit.org/changeset/234422>
All reviewed patches have been landed. Closing bug.
<rdar://problem/42840233>