Auto merge of #18373 - canaltinova:font-feature-vals, r=xidorn

Don't unnecessarily construct gfxFontFeatureValueSet

This is reviewed by xidorn in bugzilla.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [Bug 1394551](https://bugzilla.mozilla.org/show_bug.cgi?id=1394551)

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components/style/gecko/generated/bindings.rs

@@ -913,6 +913,10 @@ extern "C" {
 extern "C" {
     pub fn Gecko_nsFont_Destroy(dst: *mut nsFont);
 }
+extern "C" {
+    pub fn Gecko_ConstructFontFeatureValueSet()
+     -> *mut gfxFontFeatureValueSet;
+}
 extern "C" {
     pub fn Gecko_AppendFeatureValueHashEntry(value_set:
                                                  *mut gfxFontFeatureValueSet,
@@ -2047,10 +2051,8 @@ extern "C" {
 }
 extern "C" {
     pub fn Servo_StyleSet_BuildFontFeatureValueSet(set:
-                                                       RawServoStyleSetBorrowed,
-                                                   list:
-                                                       *mut gfxFontFeatureValueSet)
-     -> bool;
+                                                       RawServoStyleSetBorrowed)
+     -> *mut gfxFontFeatureValueSet;
 }
 extern "C" {
     pub fn Servo_StyleSet_ResolveForDeclarations(set:

components/style/stylesheets/font_feature_values_rule.rs

@@ -210,7 +210,7 @@ impl<'a, 'b, 'i, T> DeclarationParser<'i> for FFVDeclarationsParser<'a, 'b, T>
                        -> Result<(), ParseError<'i>> {
         let value = input.parse_entirely(|i| T::parse(self.context, i))?;
         let new = FFVDeclaration {
-            name: Atom::from(&*name).to_ascii_lowercase(),
+            name: Atom::from(&*name),
             value: value,
         };
         update_or_push(&mut self.declarations, new);