[webrtc-pc] state internal slot

aboba has just created a new issue for https://github.com/w3c/webrtc-pc:

== state internal slot ==
A number of objects (including IceTransport, DtlsTransport, etc.) have state attributes whose value is changed (such as when calling pc.close()).   Jan-Ivar has pointed out that since these state attributes are read-only this is problematic.  To fix this, we need to define internal slots.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1574 using your GitHub account