Add video_loopback to gn.

BUG=webrtc:5949
NOTRY=True

Review-Url: https://codereview.webrtc.org/2236473002
Cr-Original-Commit-Position: refs/heads/master@{#13748}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3ab6614d101dc6e8ff8c558bd966f9f3f05e87df
2 files changed