GitLab

The button on the right of the brightness sldier is no-op, so we should
call SetEnabled(false).
In order to do this, this CL changed disabled color of the buton icon.

TEST=manual
BUG=854148

Change-Id: I1f0e98540a86696351135275be8946b7250cbc12
Reviewed-on: https://chromium-review.googlesource.com/1107423Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568752}