Don't call CreateDtlsTransport_n from non-network thread in WebRtcSession

I'm not sure if it's possible to hit this code any more, but better
safe than sorry.

BUG=webrtc:7714
TBR=pthatcher@webrtc.org

Review-Url: https://codereview.webrtc.org/2978573002
Cr-Commit-Position: refs/heads/master@{#18960}

webrtc/pc/webrtcsession.cc

diff --git a/webrtc/pc/webrtcsession.cc b/webrtc/pc/webrtcsession.cc
index 4f17733..4ca4528 100644
--- a/webrtc/pc/webrtcsession.cc
+++ b/webrtc/pc/webrtcsession.cc
@@ -1095,7 +1095,7 @@
     bool need_rtcp = (ch->rtcp_dtls_transport() != nullptr);
     cricket::DtlsTransportInternal* rtcp_dtls_transport = nullptr;
     if (need_rtcp) {
-      rtcp_dtls_transport = transport_controller_->CreateDtlsTransport_n(
+      rtcp_dtls_transport = transport_controller_->CreateDtlsTransport(
           transport_name, cricket::ICE_CANDIDATE_COMPONENT_RTCP);
     }