stylo: Add parsing support for functional non-ts pseudoclasses, add stub -moz-system-metric pseudo

MozReview-Commit-ID: KADDH6adZqR

components/style/gecko/wrapper.rs

@@ -661,7 +661,8 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozTableBorderNonzero |
             NonTSPseudoClass::MozBrowserFrame => unsafe {
                 Gecko_MatchesElement(pseudo_class.to_gecko_pseudoclasstype().unwrap(), self.0)
-            }
+            },
+            _ => false
         }
     }