Add SctpTransport API

This exposes the "sctp" attribute on an RTCPeerConnection, with the
RTCSctpTransport type, and plumbs the surfacing of connection creation
and state changes up from the webrtc layer to the Blink layer.

Specification:
http://w3c.github.io/webrtc-pc/#rtcsctptransport-interface

Bug: chromium:818643
Change-Id: I77ad2eddcddfabd018e62ef8c5c73cfbe77ab74d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1491436
Commit-Queue: Harald Alvestrand <hta@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Henrik Boström <hbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641986}
44 files changed