Bug 244762

For content like this: ``` @font-face { font-family: a; src: url("MyFont.ttf"); unicode-range: some range; } @font-face { font-family: a; src: url("MyOtherFont.ttf"); unicode-range: a distinct range that doesn't intersect the other font's range; } ``` It's non-obvious which font should be considered the primary font for the purpose of inline layout (vertical height calculations). Right now we pick MyOtherFont.ttf just because it's last in content order, but that's wasteful because it's totally possible that none of the characters will actually be rendered with that font. We should try to redesign this in a way that: a) makes more intuitive sense b) is consistent with other browsers c) matches the spec (potentially by modifying the spec to match whatever this redesign yields)