script: Skip clearing subtree layout boxes of detached element when attaching shadow to it

[yezhizhen]

There is no layout boxes to clear in this case. For the example in #43998,
this skips the traversal many times.

Testing: Should not change visible behaviour. Try.

[mrobinson]

Do we clear layout boxes when a node is disconnected from the tree? I suspect we might not be doing that. This could lead to a situation where you attach a shadow to a removed node, put it back into the DOM, and then run into the same bug that #42237 was trying to fix.

[yezhizhen]

Do we clear layout boxes when a node is disconnected from the tree? I suspect we might not be doing that.

It seems every code path that disconnects a node guarantees layout boxes are cleared.

• All removal eventually -> Node::remove -> remove_child -> complete_remove_subtree (clears style and layout)
• All moves -> Node::move_child -> Node::complete_move_subtree (clears style and layout)