Add stefan@webrtc.org to webrtc/test/OWNERS.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1613963002 .

Cr-Original-Commit-Position: refs/heads/master@{#11339}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 693a1147c6e263165fe1e817f9ae2046f2d4dc54
1 file changed