Prepare for introduction of WebRTC rtc::PacketTransportInterface.

BUG=webrtc:6531

patch from issue 2434743002 at patchset 20001 (http://crrev.com/2434743002#ps20001)

Review-Url: https://chromiumcodereview.appspot.com/2425423008
Cr-Commit-Position: refs/heads/master@{#426750}
4 files changed