Auto merge of #17517 - canaltinova:font-lang-override, r=SimonSapin

stylo: Implement font-language-override descriptor for @font-face rule

Generally  we use computed values for `@font-face` rule descriptors but that descriptor is a bit different than the others in gecko side. It accepts strings instead of unsigned integer(which is computed value of this property). So we had to use SpecifiedValue for that in here.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1355364](https://bugzilla.mozilla.org/show_bug.cgi?id=1355364)

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components/style/gecko/rules.rs

@@ -15,6 +15,7 @@ use gecko_bindings::structs::{self, nsCSSFontFaceRule, nsCSSValue};
 use gecko_bindings::structs::{nsCSSCounterDesc, nsCSSCounterStyleRule};
 use gecko_bindings::sugar::ns_css_value::ToNsCssValue;
 use gecko_bindings::sugar::refptr::{RefPtr, UniqueRefPtr};
+use properties::longhands::font_language_override;
 use shared_lock::{ToCssWithGuard, SharedRwLockReadGuard};
 use std::{fmt, str};
 use values::generics::FontSettings;
@@ -67,6 +68,17 @@ impl ToNsCssValue for font_feature_settings::T {
     }
 }
 
+impl ToNsCssValue for font_language_override::SpecifiedValue {
+    fn convert(self, nscssvalue: &mut nsCSSValue) {
+        match self {
+            font_language_override::SpecifiedValue::Normal => nscssvalue.set_normal(),
+            font_language_override::SpecifiedValue::Override(ref lang) => nscssvalue.set_string(&*lang),
+            // This path is unreachable because the descriptor is only specified by the user.
+            font_language_override::SpecifiedValue::System(_) => unreachable!(),
+        }
+    }
+}
+
 macro_rules! map_enum {
     (
         $(