GitLab

Create an NDS for a generic/"system" profile on demand and
asynchronously to service notification requests that aren't tied to
a particular user. Apply this to WebUsbDetector, and also update
some Chrome OS specific notifications which had been doing pretty
much the same thing, but had assumed/forced synchronicity.

Bug: 783018, 793877
Change-Id: I95f4434bd6557bf846f5ed89052d4cff7f6bcf21
Reviewed-on: https://chromium-review.googlesource.com/891488
Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Jun Cai <juncai@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534729}