style: Use mozilla::Length rather than nscoord to store font sizes.

This avoids arbitrary precision loss when computing REM units and so on,
which is particularly important if we ever change the base of our app
units (but useful regardless).

Differential Revision: https://phabricator.services.mozilla.com/D79928

components/style/values/computed/font.rs

@@ -156,7 +156,7 @@ impl FontSize {
     /// Get default value of font size.
     pub fn medium() -> Self {
         Self {
-            size: NonNegative(Length::new(specified::FONT_MEDIUM_PX as CSSFloat)),
+            size: NonNegative(Length::new(specified::FONT_MEDIUM_PX)),
             keyword_info: KeywordInfo::medium(),
         }
     }