RTCDataChannel: sync with WebRTC 1.0

[aboba]

The following merged WebRTC 1.0 PRs may relate to ORTC as well:

w3c/webrtc-pc#1455 (Label/Protocol length restrictions)
w3c/webrtc-pc#1358 (RTCDataChannel use internal slots)
w3c/webrtc-pc#1356 (createDataChannel: use TypeError)
w3c/webrtc-pc#1137 (RTCDataChannel.id default value)
w3c/webrtc-pc#1131 (USVString handling)

Related:
Issue: #626
PR: #715

[lgrahl]

FYI #715 fixes w3c/webrtc-pc#1404 and w3c/webrtc-pc#1209

[aboba]

Thanks. Have removed those Issues from the list.