GitLab

Update the visibility of the the brightness row to be visible at all
time for Chrome OS MD; the visibility stays as it is for non-MD.

TEST=Tray_brightness_unittests in ash_unittests

BUG=631809

Review-Url: https://codereview.chromium.org/2329333002
Cr-Commit-Position: refs/heads/master@{#417975}