Add back P2PTransportChannel.Connect, for Chromium.

Will remove it as soon as Chromium does a roll and can stop using it.

TBR=pthatcher@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2110853005
Cr-Original-Commit-Position: refs/heads/master@{#13332}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 886815bc36ee0b8b559a29ee1d08f0ab50622917