Return correct subtype from RTCRtpSender/Receiver track.

Bug: webrtc:8915
Change-Id: Iaa004d5d3e055cdaa08daf57b662b6711ead681d
Reviewed-on: https://webrtc-review.googlesource.com/56661
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22155}
diff --git a/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm b/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
index cb2771a..0677a9b 100644
--- a/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
+++ b/sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm
@@ -62,7 +62,7 @@
   rtc::scoped_refptr<webrtc::MediaStreamTrackInterface> nativeTrack(
     _nativeRtpReceiver->track());
   if (nativeTrack) {
-    return [[RTCMediaStreamTrack alloc] initWithNativeTrack:nativeTrack];
+    return [RTCMediaStreamTrack mediaTrackForNativeTrack:nativeTrack];
   }
   return nil;
 }