Implement nack_count metric for outbound audio rtp streams.

Bug: webrtc:12510
Change-Id: Ia035885bced3c3d202bb9ffeb88c2556d4830e92
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/225021
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Reviewed-by: Erik Språng <sprang@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34444}
diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index c15ffbf..2dfe1b5 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -934,7 +934,6 @@
                                        RTCVideoSourceStats::kType);
       verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.fir_count);
       verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.pli_count);
-      verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.nack_count);
       if (*outbound_stream.frames_encoded > 0) {
         verifier.TestMemberIsNonNegative<uint64_t>(outbound_stream.qp_sum);
       } else {
@@ -943,11 +942,11 @@
     } else {
       verifier.TestMemberIsUndefined(outbound_stream.fir_count);
       verifier.TestMemberIsUndefined(outbound_stream.pli_count);
-      verifier.TestMemberIsUndefined(outbound_stream.nack_count);
       verifier.TestMemberIsIDReference(outbound_stream.media_source_id,
                                        RTCAudioSourceStats::kType);
       verifier.TestMemberIsUndefined(outbound_stream.qp_sum);
     }
+    verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.nack_count);
     verifier.TestMemberIsOptionalIDReference(
         outbound_stream.remote_id, RTCRemoteInboundRtpStreamStats::kType);
     verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.packets_sent);