devtools: Allow highlighting elements from the inspector (#35822)

This change connects the `HighlighterActor` from the devtools with the
document, which will draw a blue rectangle over any highlighted dom
node.



https://github.com/user-attachments/assets/571b2dab-497f-4102-9e55-517cdcc040ba




---
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- [X] These changes do not require tests because we don't have devtools
tests

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

components/layout/context.rs

@@ -46,6 +46,9 @@ pub struct LayoutContext<'a> {
         Arc<RwLock<FnvHashMap<(ServoUrl, UsePlaceholder), WebRenderImageInfo>>>,
 
     pub node_image_animation_map: Arc<RwLock<FxHashMap<OpaqueNode, ImageAnimationState>>>,
+
+    /// The DOM node that is highlighted by the devtools inspector, if any
+    pub highlighted_dom_node: Option<OpaqueNode>,
 }
 
 pub enum ResolvedImage<'a> {