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/fragment_tree/base_fragment.rs

@@ -13,8 +13,8 @@ use style::selector_parser::PseudoElement;
 #[derive(Clone, Debug)]
 pub(crate) struct BaseFragment {
     /// A tag which identifies the DOM node and pseudo element of this
-    /// Fragment's content. If this fragment isn't related to any DOM
-    /// node at all, the tag will be None.
+    /// Fragment's content. If this fragment is for an anonymous box,
+    /// the tag will be None.
     pub tag: Option<Tag>,
 
     /// Flags which various information about this fragment used during