GitLab

Adding automated test for set and reset custom background attribution.
The attribution should be removed when we cleared the custom background
and when a theme was applied.

Bug: 857256
Change-Id: Iccabe90925bc7e7d3da8ca0fa03705264493cf7b
Reviewed-on: https://chromium-review.googlesource.com/1208469
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589324}