When showing a non-persistent notification, the caller would receive a
closure in return that could be used to close the notification after it
had been shown. This system existed for two reasons:

  1. There was an assumption that it could outlive the profile.
  2. The notification's Id was not known to the //content layer.

Both reasons have changed -- all notification code in //content and
below is explicitly associated with a renderer, and thereby profile. The
Id is generated within //content these days.

We can therefore simplify the code by removing the closure, and instead
extending ClosePersistentNotification to work with non-persistent
notifications as well.

BUG=

Change-Id: I79530f4142c9926edab46b23cdba292c1b5e50d2
Reviewed-on: https://chromium-review.googlesource.com/744205


Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512580}