This bug is broken out from https://bugs.webkit.org/show_bug.cgi?id=49410 which uncovered two issues with our code... a negative leading issue and a usedFonts issue. I'm leaving 49410 to deal with the usedFonts issue and making thus bug to cover the negative leading problem.
Created attachment 73971 [details] Patch
Ended up combining this with 11004. *** This bug has been marked as a duplicate of bug 11004 ***