Revert "Revert "Post-process font-face rules in addRulesFromSheet.""

This reverts https://src.chromium.org/viewvc/blink?view=rev&revision=161741.
It didn't turn out to be the cause of the perf regression.

TBR=esprehn@chromium.org
BUG=311669

Review URL: https://codereview.chromium.org/66333009

git-svn-id: svn://svn.chromium.org/blink/trunk@161850 bbb929c8-8fbe-4397-9dbb-9b2b20218538