Add experimental simulcast screen content mode

webrtc/media/engine/webrtcvideoengine2.h

Index: webrtc/media/engine/webrtcvideoengine2.h
diff --git a/webrtc/media/engine/webrtcvideoengine2.h b/webrtc/media/engine/webrtcvideoengine2.h
index 1fb794eece5f78cf3b1801532a0560db0ef8d414..8f25408c065ed360de2bf2db383a7f55d4ec31e5 100644
--- a/webrtc/media/engine/webrtcvideoengine2.h
+++ b/webrtc/media/engine/webrtcvideoengine2.h
@@ -347,6 +347,8 @@ class WebRtcVideoChannel2 : public VideoMediaChannel, public webrtc::Transport {
         ACCESS_ON(&thread_checker_);
     WebRtcVideoEncoderFactory* const external_encoder_factory_
         ACCESS_ON(&thread_checker_);
+    const std::unique_ptr<WebRtcVideoEncoderFactory> internal_encoder_factory_
+        ACCESS_ON(&thread_checker_);
 
     rtc::CriticalSection lock_;
     webrtc::VideoSendStream* stream_ ACCESS_ON(&thread_checker_);