[webrtc-pc] new commits pushed by aboba

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

* Enhance the data channel's closing procedure description
Clarify that queued messages must be sent
  by Lennart Grahl
https://github.com/w3c/webrtc-pc/commit/99c487acc1429e32117f11ed4ed19079cbda596b

* Update the closing procedure to run some steps in parallel
  by Lennart Grahl
https://github.com/w3c/webrtc-pc/commit/ea927c13593a64b4a8135691e560aa334449ab8d

* Merge pull request #1822 from lgrahl/dc-closing-procedure

Data channel closing procedure
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/41468c7d67fee4459e78ffdee079d31be5b27765