style: Don't clamp font-size calc() factors too early.

These two bugs (bug 1572738 and bug 1572451) are stylo regressions.

When font-family changes, we try to recompute the font-size with a length /
percentage combinations in case the generic family changes, so the user
preferences are kept.

When calc() is involved, we clamp to non-negative too early, via
NonNegativeLength::scale_by.

I think we should generally dump this "try to track font-size across calc()"
thingie, as as various comments note it is not quite perfect, and it's not clear
how it should work in presence of min()/max().

This patch fixes the issue and simplifies code a bit, I may consider removing
this altogether in a follow-up.

Differential Revision: https://phabricator.services.mozilla.com/D41776

components/style/properties/longhands/font.mako.rs

@@ -401,7 +401,7 @@ ${helpers.predefined_type(
                         is_system_font: true,
                     },
                     font_size: FontSize {
-                        size: cx.maybe_zoom_text(Au(system.size).into()),
+                        size: NonNegative(cx.maybe_zoom_text(Au(system.size).into())),
                         keyword_info: None
                     },
                     font_weight,