Add MDnsHostLocator

This CL adds MDnsHostLocator which uses MDnsClient to discover hosts in
the local network.

Bug: chromium:757625
Change-Id: I15d449cddeaa958d27f153d323a02a3e5ad40282
Reviewed-on: https://chromium-review.googlesource.com/962982
Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Allen Vicencio <allenvic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546631}
4 files changed