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When this class was introduced, it served the needs for non-persistent
Web Notifications (and other kinds of notifications), with the knowledge
that it wouldn't be able to reliably provide many of the API methods
defined in the NotificationUIManager interface. That was OK.

Today, we only ship persistent Web Notifications, which can outlive the
browser process, and don't use the NotificationUIManagerAndroid for any
other purpose. However, the code to support other kinds of notifications
is still more or less in place, with no tests or users. It's broken.

Therefore we remove it. This CL also removes a test that doesn't make
sense anymore, since this stops us from double-closing notifications.

BUG=

Review URL: https://codereview.chromium.org/1072043003

Cr-Commit-Position: refs/heads/master@{#325273}