GitLab

If a custom background is set clear the theme attribution url.

Bug: 908798
Change-Id: I2c612fa2fffaff04925a1f8d04d6b5962b0ccc4c
Reviewed-on: https://chromium-review.googlesource.com/c/1354007Reviewed-by: Ramya Nagarajan <ramyan@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611905}