I previously believed there was no way a detached document should have
layout information, but it turns out there is a way: getComputedStyle().
So we need to account for cases where we have a layout node, but no
navigable, since that is a state we can get into at this moment.
Fixes#354
(cherry picked from commit 1e7b17f15024d0618f1a1e17e3e927856febde28)
