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Exercise showing a notification, user actions on the notification, and
closing the notification.

I'm working on lacros-side tests separately.

Bug: 1113889
Change-Id: I254ec7f50ca2bce851cb97f9cf23262902ae8c3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2350305
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798032}