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Valid DNS responses need the QR bit set to 1. See
https://tools.ietf.org/html/rfc1035#section-4.1.1

This CL adds verification to the AsyncDNS resolver to weed out invalid
responses.

R=mmenke@chromium.org

Bug: None
Change-Id: Ieda75f13394564f4617a0752d5740c9feccca850
Reviewed-on: https://chromium-review.googlesource.com/c/1292372
Commit-Queue: Asanka Herath <asanka@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602440}