The prefix was removed from WebKit in https://trac.webkit.org/changeset/202765.
<rdar://problem/27178887>
Created attachment 282809 [details] Patch
*** Bug 159434 has been marked as a duplicate of this bug. ***
Comment on attachment 282809 [details] Patch Clearing flags on attachment: 282809 Committed r202830: <http://trac.webkit.org/changeset/202830>
All reviewed patches have been landed. Closing bug.