style: Add support for the ::marker pseudo element on list items. Alias :-moz-list-bullet/number to that in the parser.

Bug: 205202
Reviewed-by: emilio

components/style/gecko/pseudo_element.rs

@@ -33,7 +33,7 @@ impl ::selectors::parser::PseudoElement for PseudoElement {
     fn valid_after_slotted(&self) -> bool {
         matches!(
             *self,
-            PseudoElement::Before | PseudoElement::After | PseudoElement::Placeholder
+            PseudoElement::Before | PseudoElement::After | PseudoElement::Marker | PseudoElement::Placeholder
         )
     }
 
@@ -180,6 +180,8 @@ impl PseudoElement {
     /// Whether this pseudo-element should actually exist if it has
     /// the given styles.
     pub fn should_exist(&self, style: &ComputedValues) -> bool {
+        debug_assert!(self.is_eager());
+
         if style.get_box().clone_display() == Display::None {
             return false;
         }

components/style/gecko/pseudo_element_definition.mako.rs

@@ -195,13 +195,16 @@ impl PseudoElement {
                 return Some(${pseudo_element_variant(pseudo)})
             }
             % endfor
-            // Alias "-moz-selection" to "selection" at parse time.
+            // Alias some legacy prefixed pseudos to their standardized name at parse time:
             "-moz-selection" => {
                 return Some(PseudoElement::Selection);
             }
             "-moz-placeholder" => {
                 return Some(PseudoElement::Placeholder);
             }
+            "-moz-list-bullet" | "-moz-list-number" => {
+                return Some(PseudoElement::Marker);
+            }
             _ => {
                 if starts_with_ignore_ascii_case(name, "-moz-tree-") {
                     return PseudoElement::tree_pseudo_element(name, Box::new([]))

components/style/gecko/wrapper.rs

@@ -1137,6 +1137,14 @@ impl<'le> TElement for GeckoElement<'le> {
         self.before_or_after_pseudo(/* is_before = */ false)
     }
 
+    fn marker_pseudo_element(&self) -> Option<Self> {
+        if !self.has_properties() {
+            return None;
+        }
+
+        unsafe { bindings::Gecko_GetMarkerPseudo(self.0).as_ref().map(GeckoElement) }
+    }
+
     #[inline]
     fn is_html_element(&self) -> bool {
         self.namespace_id() == structs::kNameSpaceID_XHTML as i32