Piggybacking simulcast id and ALR experiment id into video content type extension.

webrtc/modules/video_coding/generic_encoder.h

Index: webrtc/modules/video_coding/generic_encoder.h
diff --git a/webrtc/modules/video_coding/generic_encoder.h b/webrtc/modules/video_coding/generic_encoder.h
index a5c34d74043e523c163575539c7bd1e2e67e16ca..20e07c73784f0a0eb5a69377b7e1702f9e2d2825 100644
--- a/webrtc/modules/video_coding/generic_encoder.h
+++ b/webrtc/modules/video_coding/generic_encoder.h
@@ -82,6 +82,11 @@ class VCMEncodedFrameCallback : public EncodedImageCallback {
   int64_t last_timing_frame_time_ms_ GUARDED_BY(timing_params_lock_);
   VideoCodec::TimingFrameTriggerThresholds timing_frames_thresholds_
       GUARDED_BY(timing_params_lock_);
+
+  // Experiment groups parsed from field trials for realtime video ([0]) and
+  // screenshare ([1]). 0 means no group specified. Positive values are
+  // experiment group numbers incremented by 1.
+  uint8_t experiment_groups_[2];
 };
 
 class VCMGenericEncoder {