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..b0fc1bd37c7c98ed436aeb0b50a7a8246255af82 100644
--- a/webrtc/modules/pacing/alr_detector.h
+++ b/webrtc/modules/pacing/alr_detector.h
@@ -47,6 +47,9 @@ 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 3 bits are used and one value is reserved for N/A.
+    int group_id = 0;

[sprang_webrtc, 2017/08/24 09:13:16]

Which value means N/A? Maybe easier if 0 is undefined? Could also create a
constant for that value.

[ilnik, 2017/08/25 12:35:07]

Rephrased the comment. It doesn't matter which value will be used as N/A. All
maters is that we have 3 bits and one reserved value. It leaves 7 possible
groups here.

   };
   static rtc::Optional<AlrExperimentSettings> ParseAlrSettingsFromFieldTrial(
       const char* experiment_name);