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/specified/font.rs

@@ -787,13 +787,13 @@ impl ToComputedValue for FontSizeAdjust {
 const LARGER_FONT_SIZE_RATIO: f32 = 1.2;
 
 /// The default font size.
-pub const FONT_MEDIUM_PX: i32 = 16;
+pub const FONT_MEDIUM_PX: f32 = 16.0;
 
 impl FontSizeKeyword {
     #[inline]
     #[cfg(feature = "servo")]
     fn to_length(&self, _: &Context) -> NonNegativeLength {
-        let medium = Length::new(FONT_MEDIUM_PX as f32);
+        let medium = Length::new(FONT_MEDIUM_PX);
         // https://drafts.csswg.org/css-fonts-3/#font-size-prop
         NonNegative(match *self {
             FontSizeKeyword::XXSmall => medium * 3.0 / 5.0,

components/style/values/specified/length.rs

@@ -241,7 +241,7 @@ impl FontRelativeLength {
                 let reference_size = if context.builder.is_root_element || context.in_media_query {
                     reference_font_size
                 } else {
-                    computed::Length::new(context.device().root_font_size().to_f32_px())
+                    context.device().root_font_size()
                 };
                 (reference_size, length)
             },