Bug 1980373 - Reject @font-face's size-adjust descriptor on Servo. r=firefox-style-system-reviewers,dshin

This is a fixup for D258800, which removed the flag that was gatekeeping
this descriptor (layout.css.size-adjust.enabled). This change keeps
rejecting it on Servo, since it's not implemented.

Differential Revision: https://phabricator.services.mozilla.com/D259399

Author: Loirooriol

style/font_face.rs

@@ -576,6 +576,9 @@ macro_rules! is_descriptor_enabled {
     ("font-variation-settings") => {
         static_prefs::pref!("layout.css.font-variations.enabled")
     };
+    ("size-adjust") => {
+        cfg!(feature = "gecko")
+    };
     ($name:tt) => {
         true
     };