[webrtc-pc] new commits pushed by aboba

The following commits were just pushed by aboba to https://github.com/w3c/webrtc-pc:

* Adding more detail about RTCDataChannel.id's default value (null).

Fixes #1132.

This is an editorial change; the behavior described here was already
decided when issue #795 was discussed.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/b1bc40950f83adb84b9105eb11661cf5573a8f47

* Make RTCDataChannelInit members default to null.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/c9403b2fea23e90a856a80c25f8ca429172b92e5

* Going back to non-nullable dictionary members.

This allows the application to pass in "null", which is not what we
want.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/d985eced5f3b350ecef360b4ead34495b88c0c5e

* Adding note clarifying what RTCDataChannelInit members may be missing.
  by Taylor Brandstetter
https://github.com/w3c/webrtc-pc/commit/ef949a616d5fe1d7024d36ae4dc997047b0cdbe9

* Merge pull request #1137 from taylor-b/issue_1132_data_channel_id_null

Adding more detail about RTCDataChannel.id's default value (null).
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/d9bec381fd1a910dcb1be0b0abf062c0b002f750