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Changed addresses_ to an Optional and added additional optional fields
for text results (eg for TXT records) and hostname results (eg for SRV
or PTR records). Deliberately avoided assumptions that only one result
field could be non-nullopt at at time (but no constructor currently
allows creating it) because there's nothing wrong with some query
having results of multiple types. Eg, we may in the future choose to
pull cannonname out of AddressList and set it as a hostname result.

A subsequent CL will alter HostResolverImpl to allow querying, caching,
and returning non-address types, and it will also use HostCache::Entry
internally (and only internally) as a results container. For now, in
this CL, most of the code dealing with HostCache results will just
assume Entry::addresses() always has a value.

Bug: 846423
Change-Id: I62e177452bf89e5931f75a74448f9de70b7f2357
Reviewed-on: https://chromium-review.googlesource.com/c/1331143
Commit-Queue: Eric Orth <ericorth@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609373}