Re: [webrtc-pc] Add video latency constrainable property (#2109)

It doesn't specify *how* it should work. Implementors should not have to infer this. I think there's precedent here in https://github.com/w3c/mediacapture-screen-share/issues/31.

*MediaTrackSupportedConstraints* is returned from [navigator.mediaDevices.getSupportedConstraints()](https://w3c.github.io/mediacapture-main/getusermedia.html#dom-mediadevices-getsupportedconstraints). How exactly is it relevant to RTCPeerConnection? 

Should the following blank the remote track and  fire `overconstrainederror` and ?
```js
await transceiver.receiver.track.applyConstraints({width: {min: 1280}}); // succeeds
transceiver.receiver.track.onoverconstrainederror = () => { /* should this fire? */ };
// Network conditions or other side using replaceTrack causes lower resolution to appear
```

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