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a) if a dangerous download is cancelled, don't try to update it (with
a potentially half-destroyed profile). Like other notification types,
just close and return.

b) make the download service depend on the NotificationDisplayService.

Bug: 810302, 810738, 578868
Change-Id: Ibc3080510e128954e233485606d8de520b72a97c
Reviewed-on: https://chromium-review.googlesource.com/956087
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542010}