[webrtc-pc] Inconsistency over active member in receiver's "encoding" parameters.

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

== Inconsistency over active member in receiver's "encoding" parameters. ==
While working on https://github.com/w3c/webrtc-pc/issues/1493, I found this inconsistency over whether
```js
pc.getReceivers()[0].getParameters().encodings[0].active // is true/false or undefined
```
Receiver's [getParameters()](https://w3c.github.io/webrtc-pc/#dom-rtcrtpreceiver-getparameters) says:

*"Every member of the RTCRtpEncodingParameters dictionaries other than the RID fields is left undefined."*

whereas [active](https://w3c.github.io/webrtc-pc/#dom-rtcrtpencodingparameters-active) says:

*"For an RTCRtpReceiver, a value of true indicates that this encoding is being decoded. A value of false indicates this encoding is no longer being decoded."*

The latter seems on purpose.

Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1806 using your GitHub account