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This still lived in PlatformNotificationServiceImpl but has no reason to
be there. We can move it to the PersistentNotificationHandler, which is
far more appropriately scoped for this functionality.

Change-Id: I7b7d9001fa80c196aee2953556f0609849e39d8b
Reviewed-on: https://chromium-review.googlesource.com/1127998
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584480}