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Synchronizing displayed notifications helps to clean up already closed
notifications from the NotificationDatabase. This implements this
functionality for ChromeOS via the MessageCenter.

Bug: None
Change-Id: I660f2263451ca734887da63f0c7ab43cd12ab2b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391087
Commit-Queue: Richard Knoll <knollr@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806155}