This was added in https://bugs.webkit.org/show_bug.cgi?id=151899, but never used.
Created attachment 318302 [details] Patch
Comment on attachment 318302 [details] Patch rs=me
Comment on attachment 318302 [details] Patch Clearing flags on attachment: 318302 Committed r220872: <http://trac.webkit.org/changeset/220872>
All reviewed patches have been landed. Closing bug.
<rdar://problem/33948418>