The original code was added in bug 385263 for fixing bug 279032 that a
single font provides zero for max ascent / descent in its HHEA table
which caused Firefox to crash.
Unconditionally picking the maximum of max ascent / descent and their
em correspondents doesn't seem to be essential for working around that
case, so this patch changes it to just use the em ascent / descent when
both max ascent and descent are zero.
This fixes a webcompat problem related to Roboto font on Linux (and
presumably also Android given it uses FreeType backend as well).
MozReview-Commit-ID: EpKrfiOwnZt
--HG--
extra : rebase_source : 0619abf992fb1e1a1f3068ab172880913ebff1f1
f62e2a947a