layout: Use ServoThreadSafeLayoutNode in more places (#38626)

Use `ServoThreadSafeLayoutNode` in more places in layout rather than
`ServoLayoutNode`. The former is meant to be used during layout, but
layout 2020 was written against the latter. In general, this reduces the
amount of conversion to the thread-safe version in many places in
layout.

In addition, an unused iterator from the `script` crate
`ServoThreadSafeLayoutNodeChildrenIterator` is replaced with the child
iterator from `layout`. The `layout` version must be directly in
`script` now as it uses the dangerous variants of `next_sibling` and
`first_child`, which allow encapsulating the unsafe bits into one
module.

This will ultimately be useful for storing the layout data of
pseudo-element children of pseudo-elements properly.

Testing: This should not change any behavior and thus is covered by
existing tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>

components/script/layout_dom/element.rs

@@ -974,6 +974,20 @@ pub struct ServoThreadSafeLayoutElement<'dom> {
     pub(super) pseudo: Option<PseudoElement>,
 }
 
+impl<'dom> ServoThreadSafeLayoutElement<'dom> {
+    /// The shadow root this element is a host of.
+    pub fn shadow_root(&self) -> Option<ServoShadowRoot<'dom>> {
+        self.element
+            .element
+            .get_shadow_root_for_layout()
+            .map(ServoShadowRoot::from_layout_js)
+    }
+
+    pub fn slotted_nodes(&self) -> &[ServoLayoutNode<'dom>] {
+        self.element.slotted_nodes()
+    }
+}
+
 impl<'dom> ThreadSafeLayoutElement<'dom> for ServoThreadSafeLayoutElement<'dom> {
     type ConcreteThreadSafeLayoutNode = ServoThreadSafeLayoutNode<'dom>;
     type ConcreteElement = ServoLayoutElement<'dom>;