Implement the Servo parser for font descriptors Web API.

According to the spec, this parser should follow what we do for
@font_face rule, so we use the same type to parse the input, instead of
using the parser of their specified values.

ports/geckolib/glue.rs

@@ -77,6 +77,7 @@ use style::gecko_bindings::bindings::RawServoDeclarationBlockBorrowedOrNull;
 use style::gecko_bindings::bindings::RawServoStyleRuleBorrowed;
 use style::gecko_bindings::bindings::RawServoStyleSet;
 use style::gecko_bindings::bindings::ServoStyleContextBorrowedOrNull;
+use style::gecko_bindings::bindings::nsCSSValueBorrowedMut;
 use style::gecko_bindings::bindings::nsTArrayBorrowed_uintptr_t;
 use style::gecko_bindings::bindings::nsTimingFunctionBorrowed;
 use style::gecko_bindings::bindings::nsTimingFunctionBorrowedMut;
@@ -85,7 +86,7 @@ use style::gecko_bindings::structs::{CallerType, CSSPseudoElementType, Composite
 use style::gecko_bindings::structs::{Loader, LoaderReusableStyleSheets};
 use style::gecko_bindings::structs::{RawServoStyleRule, ServoStyleContextStrong, RustString};
 use style::gecko_bindings::structs::{ServoStyleSheet, SheetParsingMode, nsAtom, nsCSSPropertyID};
-use style::gecko_bindings::structs::{nsCSSFontFaceRule, nsCSSCounterStyleRule};
+use style::gecko_bindings::structs::{nsCSSFontDesc, nsCSSFontFaceRule, nsCSSCounterStyleRule};
 use style::gecko_bindings::structs::{nsRestyleHint, nsChangeHint, PropertyValuePair};
 use style::gecko_bindings::structs::AtomArray;
 use style::gecko_bindings::structs::IterationCompositeOperation;
@@ -4734,6 +4735,81 @@ pub extern "C" fn Servo_ParseTransformIntoMatrix(
     true
 }
 
+// https://drafts.csswg.org/css-font-loading/#dom-fontface-fontface
+#[no_mangle]
+pub extern "C" fn Servo_ParseFontDescriptor(
+    desc_id: nsCSSFontDesc,
+    value: *const nsAString,
+    data: *mut URLExtraData,
+    result: nsCSSValueBorrowedMut,
+) -> bool {
+    use cssparser::UnicodeRange;
+    use self::nsCSSFontDesc::*;
+    use style::computed_values::{font_feature_settings, font_stretch, font_style};
+    use style::font_face::{FontDisplay, FontWeight, Source};
+    use style::properties::longhands::font_language_override;
+    use style::values::computed::font::FamilyName;
+
+    let string = unsafe { (*value).to_string() };
+    let mut input = ParserInput::new(&string);
+    let mut parser = Parser::new(&mut input);
+    let url_data = unsafe { RefPtr::from_ptr_ref(&data) };
+    let context = ParserContext::new(
+        Origin::Author,
+        url_data,
+        Some(CssRuleType::FontFace),
+        ParsingMode::DEFAULT,
+        QuirksMode::NoQuirks,
+    );
+
+    macro_rules! parse_font_desc {
+        (
+            valid = [ $( $v_enum_name: ident / $t: ty, )* ]
+            invalid = [ $( $i_enum_name: ident, )* ]
+        ) => {
+            match desc_id {
+                $(
+                    $v_enum_name => {
+                        let f = match parser.parse_entirely(|i| <$t as Parse>::parse(&context, i)) {
+                            Ok(f) => f,
+                            Err(..) => return false,
+                        };
+                        result.set_from(f);
+                    },
+                )*
+                $(
+                    $i_enum_name => {
+                        debug_assert!(false, "$i_enum_name is not a valid font descriptor");
+                        return false;
+                    },
+                )*
+            }
+        }
+    }
+
+    // We implement the parser of each arm according to the implementation of @font-face rule.
+    // see component/style/font_face.rs for more detail.
+    parse_font_desc!(
+        valid = [
+            eCSSFontDesc_Family / FamilyName,
+            eCSSFontDesc_Style / font_style::T,
+            eCSSFontDesc_Weight / FontWeight,
+            eCSSFontDesc_Stretch / font_stretch::T,
+            eCSSFontDesc_Src / Vec<Source>,
+            eCSSFontDesc_UnicodeRange / Vec<UnicodeRange>,
+            eCSSFontDesc_FontFeatureSettings / font_feature_settings::T,
+            eCSSFontDesc_FontLanguageOverride / font_language_override::SpecifiedValue,
+            eCSSFontDesc_Display / FontDisplay,
+        ]
+        invalid = [
+            eCSSFontDesc_UNKNOWN,
+            eCSSFontDesc_COUNT,
+        ]
+    );
+
+    true
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn Servo_SourceSizeList_Parse(
     value: *const nsACString,