[webrtc-pc] Missing normative steps for determining codecs

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

== Missing normative steps for determining codecs ==
How codecs information is gathered, seems under-specified:

* [sender.getParamaters](http://w3c.github.io/webrtc-pc/#dom-rtcrtpsender-getparameters) says:*"The codecs sequence is populated based on the codecs that have been negotiated for sending, and which the user agent is currently capable of sending"*

* [receiver.getParamaters](http://w3c.github.io/webrtc-pc/#dom-rtcrtpreceiver-getparameters) says: *"The codecs sequence is populated based on the codecs that the receiver is currently prepared to receive"*

* And most recently this came up in https://github.com/w3c/webrtc-pc/pull/1980/commits/e4cc23c76eac027cbc783332050a6dc8f7b8daeb.

It's not clear how or when this is determined, and with the word "currently", how often results can change (can it change from under JS?)

Since `getParameters` is synchronous, perhaps this needs to be predetermined, with an internal slot changed from a queued task?  

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1982 using your GitHub account