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/dom.rs

@@ -419,12 +419,6 @@ pub trait TElement
     /// Return whether this element is an element in the HTML namespace.
     fn is_html_element(&self) -> bool;
 
-    /// Returns whether this element is a <html:slot> element.
-    fn is_html_slot_element(&self) -> bool {
-        self.get_local_name() == &*local_name!("slot") &&
-        self.is_html_element()
-    }
-
     /// Return the list of slotted nodes of this node.
     fn slotted_nodes(&self) -> &[Self::ConcreteNode] {
         &[]

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()

components/style/invalidation/element/element_wrapper.rs

@@ -295,6 +295,10 @@ impl<'a, E> Element for ElementWrapper<'a, E>
         self.element.is_html_element_in_html_document()
     }
 
+    fn is_html_slot_element(&self) -> bool {
+        self.element.is_html_slot_element()
+    }
+
     fn get_local_name(&self) -> &<Self::Impl as ::selectors::SelectorImpl>::BorrowedLocalName {
         self.element.get_local_name()
     }