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This one is a little more complicated because "AddOrUpdate" is not
sufficient --- in some cases an existing notification should be updated
but a new one should not be added.

Bug: 783018
Change-Id: I7517b7125dd6e06d7f00fcfd4a678b876c96054a
Reviewed-on: https://chromium-review.googlesource.com/772352
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517272}