Implement RTCOutboundRtpStreamStats.targetBitrate for audio.

Spec: https://w3c.github.io/webrtc-stats/#dom-rtcoutboundrtpstreamstats-targetbitrate

Bug: webrtc:13377
Change-Id: I98dd263e0b9d6e2ca94969d2a91857b14cd65f70
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/237402
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Minyue Li <minyue@webrtc.org>
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Jakob Ivarsson <jakobi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#35337}
diff --git a/pc/rtc_stats_integrationtest.cc b/pc/rtc_stats_integrationtest.cc
index 81ead00..32c567e 100644
--- a/pc/rtc_stats_integrationtest.cc
+++ b/pc/rtc_stats_integrationtest.cc
@@ -964,7 +964,6 @@
         outbound_stream.header_bytes_sent);
     verifier.TestMemberIsNonNegative<uint64_t>(
         outbound_stream.retransmitted_bytes_sent);
-    verifier.TestMemberIsUndefined(outbound_stream.target_bitrate);
     if (outbound_stream.media_type.is_defined() &&
         *outbound_stream.media_type == "video") {
       verifier.TestMemberIsDefined(outbound_stream.frames_encoded);
@@ -1000,6 +999,7 @@
       verifier.TestMemberIsNonNegative<uint32_t>(outbound_stream.frames_sent);
       verifier.TestMemberIsNonNegative<uint32_t>(
           outbound_stream.huge_frames_sent);
+      verifier.TestMemberIsUndefined(outbound_stream.target_bitrate);
       verifier.MarkMemberTested(outbound_stream.rid, true);
     } else {
       verifier.TestMemberIsUndefined(outbound_stream.frames_encoded);
@@ -1023,6 +1023,7 @@
       verifier.TestMemberIsUndefined(outbound_stream.frame_width);
       verifier.TestMemberIsUndefined(outbound_stream.frames_sent);
       verifier.TestMemberIsUndefined(outbound_stream.huge_frames_sent);
+      verifier.TestMemberIsNonNegative<double>(outbound_stream.target_bitrate);
     }
     return verifier.ExpectAllMembersSuccessfullyTested();
   }