style: Implement :defined pseudo-class for custom elements.

Bug: 1331334
Reviewed-by: emilio

components/style/gecko/selector_parser.rs

@@ -187,6 +187,9 @@ impl NonTSPseudoClass {
             NonTSPseudoClass::Fullscreen => unsafe {
                 mozilla::StaticPrefs_sVarCache_full_screen_api_unprefix_enabled
             },
+            NonTSPseudoClass::Defined => unsafe {
+                structs::nsContentUtils_sIsCustomElementsEnabled
+            },
             // Otherwise, a pseudo-class is enabled in content when it
             // doesn't have any enabled flag.
             _ => !self.has_any_flag(