It looks like WebCoreObjCExtras had more functionality which has since been removed without the includes being updated.
Created attachment 327461 [details] Patch
Comment on attachment 327461 [details] Patch Clearing flags on attachment: 327461 Committed r225100: <https://trac.webkit.org/changeset/225100>
All reviewed patches have been landed. Closing bug.
<rdar://problem/35669308>