Set screenshare.enabled parameter to false when running video_loopback test.

webrtc/video/video_loopback.cc

Index: webrtc/video/video_loopback.cc
diff --git a/webrtc/video/video_loopback.cc b/webrtc/video/video_loopback.cc
index 3de0b1411e30a40312de003cc5498e794c740e3a..1ccaf41b44eba4ade905d11378bee99eef002daa 100644
--- a/webrtc/video/video_loopback.cc
+++ b/webrtc/video/video_loopback.cc
@@ -253,6 +253,7 @@ void Loopback() {
                    flags::EncodedFramePath(),
                    call_bitrate_config};
   params.video = {flags::Clip()};
+  params.screenshare.enabled = false;
   params.analyzer = {"video", 0.0, 0.0, flags::DurationSecs(),
       flags::OutputFilename(), flags::GraphTitle()};
   params.pipe = pipe_config;