Auto merge of #20592 - upsuper:is-inherited, r=emilio

Use Servo code to check whether a property is inherited

This is the Servo side change of [bug 1452534](https://bugzilla.mozilla.org/show_bug.cgi?id=1452534).

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components/style/properties/properties.mako.rs

@@ -826,7 +826,8 @@ impl LonghandId {
         }
     }
 
-    fn inherited(&self) -> bool {
+    /// Returns whether the longhand property is inherited by default.
+    pub fn inherited(&self) -> bool {
         ${static_longhand_id_set("INHERITED", lambda p: p.style_struct.inherited)}
         INHERITED.contains(*self)
     }

ports/geckolib/glue.rs

@@ -952,6 +952,25 @@ pub unsafe extern "C" fn Servo_Property_IsShorthand(
     prop_id.is_shorthand()
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn Servo_Property_IsInherited(
+    prop_name: *const nsACString,
+) -> bool {
+    let prop_name = prop_name.as_ref().unwrap().as_str_unchecked();
+    let prop_id = match PropertyId::parse(prop_name) {
+        Ok(id) => id,
+        Err(_) => return false,
+    };
+    let longhand_id = match prop_id {
+        PropertyId::Custom(_) => return true,
+        PropertyId::Longhand(id) |
+        PropertyId::LonghandAlias(id, _) => id,
+        PropertyId::Shorthand(id) |
+        PropertyId::ShorthandAlias(id, _) => id.longhands().next().unwrap(),
+    };
+    longhand_id.inherited()
+}
+
 #[no_mangle]
 pub extern "C" fn Servo_Property_IsAnimatable(property: nsCSSPropertyID) -> bool {
     use style::properties::animated_properties;