[webrtc-pc] new commits pushed by aboba

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

* Add a note to RTCDataChannel.send to clarify which argument types are acceptable.
  by Lennart Grahl
https://github.com/w3c/webrtc-pc/commit/f9663e1b50cab1d2c5597e5d7d1c251417dd84b7

* Merge pull request #1841 from lgrahl/dc-send-null

RTCDataChannel.send argument type clarification
  by Bernard Aboba
https://github.com/w3c/webrtc-pc/commit/32d0dbfa43ba0757940941fa532dc704e393554b