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On Kiosk mode, we shouldn't show volume slider bubble. The check was
missing in UnifiedSystemTray. It caused UI regression in Kiosk devices.
(See the bug for detail).

TEST=manual(confirmed on a Kiosk device that the slider is not shown)
BUG=888890

Change-Id: I00359892b6bfc624b95c5f2a57fc7802b38a4dca
Reviewed-on: https://chromium-review.googlesource.com/1247841Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594609}