A wrapper around a LayoutDom<Node> which provides a safe interface that
can be used during layout. This implements the LayoutNode trait as well as
several style and selectors traits for use during layout. This version
should only be used on a single thread. If you need to use nodes across
threads use ServoThreadSafeLayoutNode.
A wrapper around a ServoLayoutNode that can be used safely on different threads.
It’s very important that this never mutate anything except this wrapped node and
never access any other node apart from its parent.