GitLab

The z order of an underlay should be a decreasing number for each
underlay layer at the bottom. It doesn't have to be a contiguous
number though.

Bug: 868561
Change-Id: I3ba9afbd9f02404ca23f416a464f18c30771b962
Reviewed-on: https://chromium-review.googlesource.com/c/1488151
Commit-Queue: Maggie Chen <magchen@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635870}