layout: Introduce infrastructure for tracking, backing up, and splitting at the last known good split point, and use it for `white-space: nowrap`.
Fixes overflowing tables on Wikipedia.
This infrastructure should form the basis of our fix for inline layout
of fragments that don't themselves constitute valid split points. That
will require some more work, however.
r? @mbrubeck
<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/7056)
<!-- Reviewable:end -->
d07cde5efc
