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

webrtc/modules/video_coding/generic_encoder.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 64 matching lines @@
     size_t target_bitrate_bytes_per_sec = 0;
     std::map<int64_t, int64_t> encode_start_time_ms;
   };
   // Separate instance for each simulcast stream or spatial layer.
   std::vector<TimingFramesLayerInfo> timing_frames_info_
       GUARDED_BY(timing_params_lock_);
   size_t framerate_ GUARDED_BY(timing_params_lock_);
   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];

[philipel, 2017/08/29 12:06:15]

I think you will only need an uint8_t since the both experiments can't be active
at the same time.

[ilnik, 2017/08/29 12:41:22]

I think no, because a single VCMReceiverCallback may is used for several
streams. It's safer to take screenshare status directly from the frame in the
callback.

 };
 
 class VCMGenericEncoder {
   friend class VCMCodecDataBase;
 
  public:
   VCMGenericEncoder(VideoEncoder* encoder,
                     VCMEncodedFrameCallback* encoded_frame_callback,
                     bool internal_source);
   ~VCMGenericEncoder();
@@ skipping 22 matching lines @@
   const bool internal_source_;
   rtc::CriticalSection params_lock_;
   EncoderParameters encoder_params_ GUARDED_BY(params_lock_);
   bool is_screenshare_;
   size_t streams_or_svc_num_;
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_VIDEO_CODING_GENERIC_ENCODER_H_