Created attachment 56905 [details]
Re-commit r58765 - it had been rolled out to see if it was causing a perf regression (in r59787 and r59789), but that does not seem to have been the case.
https://bugs.webkit.org/show_bug.cgi?id=38336 tracked the original bug ; https://bugs.webkit.org/show_bug.cgi?id=3928 tracked the revert.
Comment on attachment 56905 [details]
Committed r60193: <http://trac.webkit.org/changeset/60193>