layout: Simplify and generalize the usage of pseudo-elements (#36202)

- Remove the last remaining Servo-specific PseudoElement enum from
  layout. This was made to select `::before` and `::after` (both eager
  pseudo-elements), but now `traverse_pseudo_element` is called
  `traverse_eager_pseudo_element` and should work on any eager pseudo
  element.
- Expose a single way of getting psuedo-element variants of
  ThreadSafeLayoutElement in the Layout DOM, which returns `None` when
  the pseudo-element doesn't apply (not defined for eager
  pseudo-elements or when trying to get `<details>` related
  pseudo-elements on elements that they don't apply to).
- Ensure that NodeAndStyleInfo always refers to a node. This is done by
  making sure that anonymous boxes are all associated with their
  originating node.

These changes are prepatory work for implementation of the `::marker`
pseudo-element as well as ensuring that all anonymous boxes can be
cached into the box tree eventually.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/layout_2020/query.rs

@@ -116,19 +116,19 @@ pub fn process_resolved_style_request<'dom>(
     // We call process_resolved_style_request after performing a whole-document
     // traversal, so in the common case, the element is styled.
     let layout_element = node.to_threadsafe().as_element().unwrap();
-    let layout_element = pseudo.map_or_else(
-        || Some(layout_element),
-        |pseudo_element| layout_element.get_pseudo(pseudo_element),
-    );
-
-    let layout_element = match layout_element {
-        None => {
-            // The pseudo doesn't exist, return nothing.  Chrome seems to query
-            // the element itself in this case, Firefox uses the resolved value.
-            // https://www.w3.org/Bugs/Public/show_bug.cgi?id=29006
-            return String::new();
+    let layout_element = match pseudo {
+        Some(pseudo_element_type) => {
+            match layout_element.with_pseudo(*pseudo_element_type) {
+                Some(layout_element) => layout_element,
+                None => {
+                    // The pseudo doesn't exist, return nothing.  Chrome seems to query
+                    // the element itself in this case, Firefox uses the resolved value.
+                    // https://www.w3.org/Bugs/Public/show_bug.cgi?id=29006
+                    return String::new();
+                },
+            }
         },
-        Some(layout_element) => layout_element,
+        None => layout_element,
     };
 
     let style = &*layout_element.resolved_style();