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Allows DnsClient::CancelProbesForContext() to cleanly cancel probes
when still waiting on initial startup delay. Nulling the
|url_request_context_for_probes_| was not sufficient for this case
because the context was already bound in the delayed task.

For testability, moved MockDnsTransactionFactory to be visible from
dns_test_util.h instead of just being a private internal class of
MockDnsClient.  Allows use with a real DnsClient for DnsClientTests.
Bunch of code churn from moving, but no real change to
MockDnsTransactionFactory other than adding a |doh_probes_running_| to
track when the probes have been started or canceled.

Fixed: 1015555
Change-Id: I09a23bc3e90267723d76f727be3bf5390ad6ca37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1867443Reviewed-by: Matt Menke <mmenke@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707664}