Socket broker classes are now Windows only.

Since socket brokering is only required on windows now, this CL makes
the relevant classes win only instead of platform agnostic.

BrokeredUdpClientSocket still has functionality for connecting
with a network handle, that will be removed in a follow up CL to
simplify connecting code.

Bug: 1485298
Change-Id: I7a05ccaed865e9fa99dcea7399379d86dbdf35e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4897359
Reviewed-by: Brendon Tiszka <tiszka@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Liza Burakova <liza@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1204744}