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Only used when explicitly requested. The MDnsClient is created and
starts listening for MDNS messages at first use in HostResolverImpl.

For now supports only a basic feature set, eg we'll DCHECK if canonname
is requested (in contrast to the async resolver behavior that was
unnoticed until recently due to the lack of a DCHECK).

Bug: 846423
Cq-Include-Trybots: luci.chromium.try:linux_mojo
Change-Id: Ib296ac432d392baee2454587693b2537513f461c
Reviewed-on: https://chromium-review.googlesource.com/1211542Reviewed-by: Matt Menke <mmenke@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595956}