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/mod.rs

@@ -232,7 +232,7 @@ impl<'a> Context<'a> {
         // -x-text-zoom property, which leads to a false value
         // in mAllowZoomAndMinSize
         if self.style().get_font().gecko.mAllowZoomAndMinSize {
-            self.device().zoom_text(Au::from(size)).into()
+            self.device().zoom_text(size)
         } else {
             size
         }