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To send DoH requests, LogDnsClient needs to set the URLRequestContext
to be used for a transaction. The TreeStateTracker is configured with
a URLRequestContext that is passed to the resulting LogDnsClient.

Bug: 878582
Change-Id: Iea5be660ac1ac0db9a2338386201ff6a92796b0a
Reviewed-on: https://chromium-review.googlesource.com/c/1486718
Commit-Queue: Katharine Daly <dalyk@google.com>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635697}