DevTools: update layout test docs for debugging DevTools tests

Bug: 667560
Change-Id: Ic335a8364393e85de100e5b82f2d03dd6983ff84
Reviewed-on: https://chromium-review.googlesource.com/801258
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Commit-Queue: Will Chen <chenwilliam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#520699}
1 file changed