After commit ff48b7333c, we remove shadow
roots from elements that are removed from the DOM. Setting a node's
shadow root to null also sets that shadow root's host to null. Thus, the
comment in Node::is_shadow_including_descendant_of that assumes the host
is always non-null is not true.
The test added here would previously crash when interacting with a node
that is a descendant of a removed shadow root.
