Pass AI_DNS_ONLY for proxy DNS requests on Windows

On Windows, proxy autoconfiguration is enabled by default and can block
most URL requests while being checked. Reduce the impact of these checks
on the common no-proxy case by disabling resolution protocols that are
very slow to get negative results. This parameter should match the
behavior of Windows/Microsoft proxy code.

Also, while adding a new HostResolverFlag to control the param, removed
an old disused flag (HOST_RESOLVER_SYSTEM_ONLY).

Bug: 1176970
Change-Id: I36027b4949b93897416d162d7c878b486b7475e2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2704905
Reviewed-by: Dan McArdle <dmcardle@chromium.org>
Commit-Queue: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#856452}