selectors: Never match ::slotted on <slot>s.

This fixes the test from https://github.com/w3c/web-platform-tests/pull/9212 in
Gecko.

components/style/gecko/wrapper.rs

@@ -2205,6 +2205,12 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         self.as_node().owner_doc().is_html_document()
     }
 
+    #[inline]
+    fn is_html_slot_element(&self) -> bool {
+        self.is_html_element() &&
+        self.get_local_name().as_ptr() == local_name!("slot").as_ptr()
+    }
+
     #[inline]
     fn ignores_nth_child_selectors(&self) -> bool {
         self.is_root_of_anonymous_subtree()