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Its only responsibility is to forward calls to a NotificationHandler of
a specific type, which should instead be done as an implementation
detail of the notification system.

Bug: 
Change-Id: Ia7e541c4b789b68d61209647294e420f9cde0681
Reviewed-on: https://chromium-review.googlesource.com/806220
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521748}