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As described in the design doc, old SystemTray class is going to be
replaced by UnifiedSystemTray class. Tray icons will be implemented in
UnifiedSystemTray.

This CL adds notification counter and do-not-disturb icon to
UnifiedSystemTray button.

Screenshot:
  * Counter: http://screen/cgktiihNYDo
  * Mute icon: http://screen/613vrQB3suz
UX spec: http://shortn/_4BzEvhLxJ5
Design doc: go/cros-qs-restyling

TEST=manual
BUG=836134

Change-Id: Ia8c75290afe6c63a70125808d6ebd9bc7b0c37b9
Reviewed-on: https://chromium-review.googlesource.com/1060779Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#560267}