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This patch adds classes to implement the mDNS interfaces
that Open Screen expects, using the discovery system inside of
Chromium.

Change-Id: I450b757241ff99a59ec59f5352a0b604f729b9db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1590536
Commit-Queue: Jordan Bayles <jophba@chromium.org>
Reviewed-by: mark a. foltz <mfoltz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664404}