Convert DNS results extractor to use new internal results type

HostResolverManager then handles the new type for now by immediately
converting to the old type.  Will convert that in a later CL.

Also, a bit of modernization in the extractor interface:
* Pass references instead of pointers since there's no reason to pass
  nullptr.
* Use base::expected instead of out params for all the error-or-result
  returns.

Minor behavior change, not expected to have any real-world effect:
* Alias parsing now rejects aliases that do not pass a
  dns_names_util::IsValidDnsRecordName() check. This should not affect
  any real-world aliases because it's unlikely a DNS server would ever
  send us such results, and if they ever did, it's a completely
  nonsensical result anyway.

Bug: 1381506
Change-Id: Ib5f6ee4a0fcee05229252ae91b1a1c75d0fd6c40
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4504471


Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1146536}