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- All non-persistent notification events (onshow, onclick, and
onclose) now go through Mojo when the NotificationsWithMojo
blink feature flag is enabled.

- Layout tests with mojo enabled that rely on
 non-persistent notification events now pass and are re-enabled.
(Except for those relying on the Notification.close() path which
still needs to be migrated to mojo).

- Interactive ui tests with mojo enabled are simplified to wait
 for the show event rather than NotificationDisplayService
 mutation.

BUG=796990

Change-Id: Id99cd7b936b2ef18fd4be270aa76e4732e137db9
Reviewed-on: https://chromium-review.googlesource.com/878744Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Anita Woodruff <awdf@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531953}