Move secure DNS mode logic into HostResolverManager from DnsTransaction.

This change is a precursor to a follow-up change where secure and insecure
cache lookups will occur separately. Secure lookups and insecure lookups
are now separated into separate DnsTasks and cannot be combined in a result.

This change also prevents SECURE mode from falling back to ProcTask.

Bug: 878582
Change-Id: I7f5b1f5bcb1c205575c7c28bb066c55678fea466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1529731
Commit-Queue: Katharine Daly <dalyk@google.com>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665150}
16 files changed