Re: [webrtc-pc] `ssrc` in `RTCRtpEncodingParameters` is inconsistent with ORTC (#1174)

@amithilbuch my use case is adding a transceiver with multiple encodings for enabling simulcast to talk to an SFU:

```js
pc.addTransceiver(videoTrack,{
	direction	: "sendonly",
	streams		: [stream],
	sendEncodings	: [
        	{"rid":"a"},
                {"rid":"b","scaleDownResolutionBy":2},	
                {"rid":"c","scaleDownResolutionBy":4}
        ]
});
```
`rid` is not set correcty and `pc.getTransceivers()[0].sender.getParameters().encodings` values are not set:
```
[{"active":true,"networkPriority":"low","priority":"low"},
{"active":true,"networkPriority":"low","priority":"low"},
{"active":true,"networkPriority":"low","priority":"low"}]
```
and in SDP are also not set:
```
a=rid:0 send
a=rid:1 send
a=rid:2 send
a=simulcast:send 0;1;2
```
There is a CL already that supposedly implements this and I hope it lands before getting to beta. 



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