GitLab

Applies to the horizontal divider line under the folder heading, and
above the contents switcher view, and the folder back button. Also set
the contents switcher background colour to the main background colour
(it doesn't make sense to have a different colour if the divider line
doesn't reach all the way to the edge).

BUG=411775

Review URL: https://codereview.chromium.org/578223002

Cr-Commit-Position: refs/heads/master@{#295710}