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The order of the stacked notifications icons can be broken if a
notification is added / removed from the middle of the stacked
notifications. This fix prevents it by removing a notification's icon
through id lookup when a notification is removed and resetting the
notification_icons_container_ if a new notification is added while the
tray is open.

Change-Id: I108c983dcb1b42be67354598064e5e1daa299697
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1913877
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@{#715117}