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Instead of continuing to wait for UDP data, fail the DNS query in
DnsUDPAttempt.  The motivation is to call the callback at most once, to
conform to //net convention.  See
https://docs.google.com/document/d/1CNq4plxEbprzXwkeIrEtwM4b-Na2ZOqw8UHPRK3t-1w
for analysis of data (restricted view).

Net.DNS.ResultAfterMalformedResponse histogram is removed in
follow-up CL https://crrev.com/c/1149112.

Bug: 779589
Change-Id: I0ef3c460e40cefe8160334cad17f9ef9abec3b39
Reviewed-on: https://chromium-review.googlesource.com/1146878
Commit-Queue: Bence Béky <bnc@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577761}