NullSocketServer needs to be exported in order to use it in
JingleThreadWrapper in chromium.
Bug: none
Change-Id: I9bce49c764a1ca1c28fc44041d0d5f04f794066e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/173920
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#31174}