[LayoutNG] Make percent sizing work through anonymous constraint spaces.

Re-enable a unit test that now passes again. Also fixes a layout test.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: Ic1d15dccaefc5773ae4305771808d328548007b4
Reviewed-on: https://chromium-review.googlesource.com/637910
Commit-Queue: Morten Stenshorne <mstensho@opera.com>
Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498034}