This appears to have been introduced somewhere around r185016.
<rdar://problem/21511122>
Created attachment 255898 [details] Patch
Comment on attachment 255898 [details] Patch r=me
Comment on attachment 255898 [details] Patch Clearing flags on attachment: 255898 Committed r186181: <http://trac.webkit.org/changeset/186181>
All reviewed patches have been landed. Closing bug.