[mediacapture-image] Does ImageCapture work on non-camera tracks? (#272)

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

== Does ImageCapture work on non-camera tracks? ==
ImageCapture's [constructor](https://w3c.github.io/mediacapture-image/#dom-imagecapture-imagecapture) takes a `videoTrack` arg, making it a sink in the MediaStreamTrack model of [sources and sinks](https://docs.google.com/presentation/d/10myL8D_a3oQghAH1RO522x00KO4qmGi9ZQyXQD70Xto#slide=id.ga38008d958_3_120).

But this seems to make little sense outside camera tracks. E.g.
```js
const pc = new RTCPeerConnection();
new ImageCapture(pc.addTransceiver("video").receiver.track).takePhoto({redEyeReduction: true});
```

 What's supposed to happen, and why allow it? This is a concern for cross-origin protections.


Please view or discuss this issue at https://github.com/w3c/mediacapture-image/issues/272 using your GitHub account


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