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This CL implements a counter view at the top of UnifiedSystemTray that
shows number of notifications hidden by scrolling.

Screenshot: http://screen/LK1ETwVGn2d

BUG=863635

Change-Id: I4cc16ec8f7e5f1f0b64836b1217655a52ca9882e
Reviewed-on: https://chromium-review.googlesource.com/1168945Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582485}