This is caused by sandbox violations and was introduced in r240500.
rdar://problem/47974770
Created attachment 361864 [details] Patch
Comment on attachment 361864 [details] Patch r=me
Comment on attachment 361864 [details] Patch Thanks for reviewing!
Comment on attachment 361864 [details] Patch Clearing flags on attachment: 361864 Committed r241332: <https://trac.webkit.org/changeset/241332>
All reviewed patches have been landed. Closing bug.