Add jitterBufferTargetDelay as RTCNonStandardStatsMember to new GetStats API

Bug: webrtc:11381
Change-Id: I7df3450e50da49d178e1e3a5d9f4970672d91aac
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/169120
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Ivo Creusen <ivoc@webrtc.org>
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#30758}

modules/audio_coding/include/audio_coding_module_typedefs.h

@@ -90,6 +90,8 @@ struct NetworkStatistics {
   uint64_t concealmentEvents;
   uint64_t jitterBufferDelayMs;
   uint64_t jitterBufferEmittedCount;
+  // Non standard stats propagated to spec complaint GetStats API.
+  uint64_t jitterBufferTargetDelayMs;
   uint64_t insertedSamplesForDeceleration;
   uint64_t removedSamplesForAcceleration;
   uint64_t fecPacketsReceived;