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This allows us to get rid of an ugly priority promotion hack used for
download notifications on Chrome OS.

On platforms with a message center bubble (Chrome OS), web
notifications that don't specify renotify won't pop
up again if the original popup was dismissed.

On platforms without a message center bubble (Linux, Windows when
using non-native notifications), all notifications will pop up again
on update.

Bug: none
Change-Id: I6a00b21348335b29a97a78a603c03a31ae8e5452
Reviewed-on: https://chromium-review.googlesource.com/976545Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Yoshiki Iguchi <yoshiki@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552424}