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

webrtc/modules/video_coding/generic_encoder.cc

Index: webrtc/modules/video_coding/generic_encoder.cc
diff --git a/webrtc/modules/video_coding/generic_encoder.cc b/webrtc/modules/video_coding/generic_encoder.cc
index 78eeef288a2e7046c00adc934d438769ab9177a3..a134b774f01ef6cd745cadcc4ee569b280462819 100644
--- a/webrtc/modules/video_coding/generic_encoder.cc
+++ b/webrtc/modules/video_coding/generic_encoder.cc
@@ -13,6 +13,7 @@
 #include <vector>
 
 #include "webrtc/api/video/i420_buffer.h"
+#include "webrtc/modules/pacing/alr_detector.h"
 #include "webrtc/modules/video_coding/encoded_frame.h"
 #include "webrtc/modules/video_coding/media_optimization.h"
 #include "webrtc/rtc_base/checks.h"
@@ -20,6 +21,7 @@
 #include "webrtc/rtc_base/optional.h"
 #include "webrtc/rtc_base/timeutils.h"
 #include "webrtc/rtc_base/trace_event.h"
+#include "webrtc/system_wrappers/include/field_trial.h"
 
 namespace webrtc {
 
@@ -294,6 +296,24 @@ EncodedImageCallback::Result VCMEncodedFrameCallback::OnEncodedImage(
     encoded_image.SetEncodeTime(*encode_start_ms, rtc::TimeMillis());
   }
 
+  // Piggyback ALR experiment group id and simulcast id into the content type.
+  uint8_t experiment_id = 0;
+  rtc::Optional<AlrDetector::AlrExperimentSettings> experiment_settings =
+      AlrDetector::ParseAlrSettingsFromFieldTrial();
+  if (experiment_settings) {
+    // 0 would mean no experiment, therefore adding 1. It will be subtracted at
+    // the receive side before reporting fake field trial.
+    experiment_id = experiment_settings->group_id + 1;
+  }
+
+  // TODO(ilnik): This will force content type extension to be present even
+  // for realtime video. At the expense of miniscule overhead we will get
+  // sliced receive statistics.
+  encoded_image.content_type_.SetExperimentId(experiment_id);
+  // We number simulcast streams from 1 on the network.
+  encoded_image.content_type_.SetSimulcastId(
+      static_cast<uint8_t>(simulcast_svc_idx + 1));
+
   Result result = post_encode_callback_->OnEncodedImage(
       encoded_image, codec_specific, fragmentation_header);
   if (result.error != Result::OK)