Cheat the system and use an Arc in BoxSlot

Ideally we want to be able to borrow from the DOM during box construction
but that's not playing very well at the moment with the current bridge
between script and layout.

components/layout_2020/flow/construct.rs

@@ -178,7 +178,7 @@ impl BlockContainer {
     }
 }
 
-impl<'dom, Node> TraversalHandler<Node> for BlockContainerBuilder<'dom, '_, Node>
+impl<'dom, Node> TraversalHandler<'dom, Node> for BlockContainerBuilder<'dom, '_, Node>
 where
     Node: NodeExt<'dom>,
 {
@@ -187,7 +187,7 @@ where
         style: &Arc<ComputedValues>,
         display: DisplayGeneratingBox,
         contents: Contents<Node>,
-        box_slot: BoxSlot,
+        box_slot: BoxSlot<'dom>,
     ) {
         match display {
             DisplayGeneratingBox::OutsideInside { outside, inside } => match outside {