Only create no-op DTLS if media transport is used for both media and data

Currently it's possible that no-op DTLS is created if media transport is only used for data channels.
Changing it so that no-op DTLS is only created when both media & data will flow through media transport.

Bug: webrtc:9719
Change-Id: I87f27fc778ea21b12f2904bad1452d893f66b541
Reviewed-on: https://webrtc-review.googlesource.com/c/119909
Commit-Queue: Peter Slatala <psla@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Reviewed-by: Anton Sukhanov <sukhanov@webrtc.org>
Reviewed-by: Bjorn Mellem <mellem@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26416}
diff --git a/api/peer_connection_interface.h b/api/peer_connection_interface.h
index 3115e25..a429519 100644
--- a/api/peer_connection_interface.h
+++ b/api/peer_connection_interface.h
@@ -602,9 +602,9 @@
     bool active_reset_srtp_params = false;
 
     // If MediaTransportFactory is provided in PeerConnectionFactory, this flag
-    // informs PeerConnection that it should use the MediaTransportInterface.
-    // It's invalid to set it to |true| if the MediaTransportFactory wasn't
-    // provided.
+    // informs PeerConnection that it should use the MediaTransportInterface for
+    // media (audio/video). It's invalid to set it to |true| if the
+    // MediaTransportFactory wasn't provided.
     bool use_media_transport = false;
 
     // If MediaTransportFactory is provided in PeerConnectionFactory, this flag