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With this change each notification that is in the message center
but not visible will have an icon representing it in the stacking bar.
The icons are removed as we scroll up through the message center.
If there is not enough space to show all icons there will be a
"+ x" appended after the icons to signify remaining notifications.

Bug: 1002714
Change-Id: I2e7737a4cf0dd145e0868e7fbf11a15be897d51d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1803731
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702203}