Unified: Apply same anchor insets to volume slider

This CL applies same anchor insets to both UnifiedSystemTray bubble
and volume slider bubble. It gives 8dip padding between the shelf and
the volume slider (see the screenshot in the bug).

TEST=manual
BUG=888182

Change-Id: Idd455bb272cf58fb023a69359cd6ec68755a2b59
Reviewed-on: https://chromium-review.googlesource.com/1250461
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595712}