Fix resolved value of font-size.
Deriving To* generally assumes that if the type is not generic you can
just clone. This doesn't hold for FontSize.
Differential Revision: https://phabricator.services.mozilla.com/D212904
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1901196
gecko-commit: b9c463c892ca779258d97787a3893630cfd60304
gecko-reviewers: firefox-style-system-reviewers, zrhoffman