Add success field to FindHostCallback

This CL adds a bool success field to FindHostCallback to indicate if the
HostLocator was successful in querying for hosts.

Bug: chromium:757625
Change-Id: Ie1a2c4ee0c8392157234cc1f1eefdb0d3bc4d88f
Reviewed-on: https://chromium-review.googlesource.com/963757
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545155}
6 files changed