GitLab

As UX spec is now ready, this CL updates the style of slider bubbles
e.g. volume slider and brightness slider.

TEST=manual
BUG=870473

Change-Id: I8c3ca92e6fd2f8d834c494144578bf06d78276b2
Reviewed-on: https://chromium-review.googlesource.com/1166779
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581808}