Fix #1435: em units in font-size

Font-based units in the font-size property refer to the font-size
of the parent, not itself.
Add a test for percentages in font-size
auto merge of #1436 : SimonSapin/servo/font-size-em, r=kmcallister

Font-based units in the font-size property refer to the font-size of the parent, not itself.