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

webrtc/modules/pacing/alr_detector.h

Index: webrtc/modules/pacing/alr_detector.h
diff --git a/webrtc/modules/pacing/alr_detector.h b/webrtc/modules/pacing/alr_detector.h
index 0dcd9effd006c4343e8b2f6e05850cab20004b2a..1e124d99e919d476205723638bce58b3d7329113 100644
--- a/webrtc/modules/pacing/alr_detector.h
+++ b/webrtc/modules/pacing/alr_detector.h
@@ -47,6 +47,10 @@ class AlrDetector {
     int alr_bandwidth_usage_percent = kDefaultAlrBandwidthUsagePercent;
     int alr_start_budget_level_percent = kDefaultAlrStartBudgetLevelPercent;
     int alr_stop_budget_level_percent = kDefaultAlrStopBudgetLevelPercent;
+    // Will be sent to the receive side for stats slicing.
+    // Can be 0..6, because it's sent as a 3 bits value and there's also
+    // reserved value to indicate absence of experiment.
+    int group_id = 0;
   };
   static rtc::Optional<AlrExperimentSettings> ParseAlrSettingsFromFieldTrial(
       const char* experiment_name);