[LayoutStability] Move layout tests to WPT

Bug: 581518
Change-Id: I475757c9dc66e192275b1ae02353f15a1fc8f926
Reviewed-on: https://chromium-review.googlesource.com/c/1468581
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631861}
4 files changed