style: Simplify calc expressions earlier.

This simplifies a bit the code, and guarantees that all calc()s have percentages
and lengths.

I also wanted to remove unclamped_length() / specified_percentage() (for the
same reason as the above patch), but they're needed for animations for now. When
I implement min() / max() for <length-percentage> they'll be fixed.

Differential Revision: https://phabricator.services.mozilla.com/D60194

components/style/values/specified/text.rs

@@ -97,7 +97,7 @@ impl ToComputedValue for LineHeight {
                         let computed_calc =
                             calc.to_computed_value_zoomed(context, FontBaseSize::CurrentStyle);
                         let base = context.style().get_font().clone_font_size().size();
-                        computed_calc.percentage_relative_to(base)
+                        computed_calc.resolve(base)
                     },
                 };
                 GenericLineHeight::Length(result.into())