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Right now, this code will hide the notification at the end of the
timeout, even we switched to showing a different article in the
notification in the meantime. Fixing that will mean switching to
different tags for different articles.

Notifications should also implicitly be hidden if the articles
themselves expire, since the notifier service will watch for their
removal.

BUG=675561

Review-Url: https://codereview.chromium.org/2616893003
Cr-Commit-Position: refs/heads/master@{#442010}