Re: [mediacapture-main] getUserMedia can be used to detect capabilities without permissions (#697)

We've discussed this in the past, and we decided trackers would not risk a prompt.

In the [spec](https://w3c.github.io/mediacapture-main/getusermedia.html#dom-mediadevices-getusermedia), we neutered the `error.contraint` property ahead of permission to mitigate the attack where multiple failing constraints are used: *"Run the ApplyConstraints algorithm on all tracks in stream with the appropriate constraints. Should this fail, let failedConstraint be the result of the algorithm that failed if device information can be exposed is true, or undefined otherwise"*

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