Experiment for the nack module.

Testing the nack module by implementing it into the current jitter buffer
under the experiment WebRTC-NewVideoJitterBuffer.

BUG=webrtc:5514

Review URL: https://codereview.webrtc.org/1778503002

Cr-Commit-Position: refs/heads/master@{#11969}
diff --git a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
index 86ed6ce..e67b635 100644
--- a/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
+++ b/webrtc/modules/rtp_rtcp/source/rtp_rtcp_impl.cc
@@ -742,8 +742,14 @@
   }
   nack_last_seq_number_sent_ = nack_list[start_id + nack_length - 1];
 
-  return rtcp_sender_.SendRTCP(
-      GetFeedbackState(), kRtcpNack, nack_length, &nack_list[start_id]);
+  return rtcp_sender_.SendRTCP(GetFeedbackState(), kRtcpNack, nack_length,
+                               &nack_list[start_id]);
+}
+
+void ModuleRtpRtcpImpl::SendNack(
+    const std::vector<uint16_t>& sequence_numbers) {
+  rtcp_sender_.SendRTCP(GetFeedbackState(), kRtcpNack, sequence_numbers.size(),
+                        sequence_numbers.data());
 }
 
 bool ModuleRtpRtcpImpl::TimeToSendFullNackList(int64_t now) const {