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This is a reland of the original fix (see bug), which was reverted due to
regressions.

This updated version fixes an edge case, which caused a crash when dismissing
the message center via accelerator in certian circumstances.

BUG=1024100

Change-Id: I525186a7075732946403ebffa5def4631bcde658
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2013525Reviewed-by: Ahmed Mehfooz <amehfooz@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733852}