style: Implement GeckoElement::assigned_slot.

This allows to selector-match ::slotted, though we still don't parse it.

Bug: 1425755
Reviewed-by: heycam
MozReview-Commit-ID: ItELHkf2PMl

components/style/gecko/wrapper.rs

@@ -1795,11 +1795,21 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         parent_node.and_then(|n| n.as_element())
     }
 
+    #[inline]
     fn pseudo_element_originating_element(&self) -> Option<Self> {
         debug_assert!(self.implemented_pseudo_element().is_some());
         self.closest_non_native_anonymous_ancestor()
     }
 
+    #[inline]
+    fn assigned_slot(&self) -> Option<Self> {
+        let slot = self.get_extended_slots()?._base.mAssignedSlot.mRawPtr;
+
+        unsafe {
+            Some(GeckoElement(&slot.as_ref()?._base._base._base._base))
+        }
+    }
+
     #[inline]
     fn first_child_element(&self) -> Option<Self> {
         let mut child = self.as_node().first_child();

components/style/invalidation/element/element_wrapper.rs

@@ -351,6 +351,11 @@ impl<'a, E> Element for ElementWrapper<'a, E>
             .map(|e| ElementWrapper::new(e, self.snapshot_map))
     }
 
+    fn assigned_slot(&self) -> Option<Self> {
+        self.element.assigned_slot()
+            .map(|e| ElementWrapper::new(e, self.snapshot_map))
+    }
+
     fn blocks_ancestor_combinators(&self) -> bool {
         self.element.blocks_ancestor_combinators()
     }