Switch addDiscoveredPrinter to returning a promise

This change switches addDiscoveredPrinter to returning a promise.
Since adding a discovered printer can take a bit of time, we continue
to use the configuring dialog.

Change-Id: I22b8f052cb1786200044473cac5c60c5840483c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1649193
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#671791}
6 files changed