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-Original-Commit-Position: refs/heads/master@{#18960}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d8cf08f1663c195cc4ed7ba60b3fa6cfec82e091
1 file changed