Only generate one CNAME per PeerConnection.

The CNAME is generated in the PeerConnection constructor and is populated through the MediaSessionOptions.
A default cname will be set in the MediaSessionOptions constructor.

BUG=webrtc:3431

Review-Url: https://codereview.webrtc.org/1871993002
Cr-Commit-Position: refs/heads/master@{#12650}
diff --git a/webrtc/api/peerconnection.h b/webrtc/api/peerconnection.h
index b557715..862c6fb 100644
--- a/webrtc/api/peerconnection.h
+++ b/webrtc/api/peerconnection.h
@@ -369,6 +369,10 @@
   std::unique_ptr<cricket::PortAllocator> port_allocator_;
   std::unique_ptr<MediaControllerInterface> media_controller_;
 
+  // One PeerConnection has only one RTCP CNAME.
+  // https://tools.ietf.org/html/draft-ietf-rtcweb-rtp-usage-26#section-4.9
+  std::string rtcp_cname_;
+
   // Streams added via AddStream.
   rtc::scoped_refptr<StreamCollection> local_streams_;
   // Streams created as a result of SetRemoteDescription.