getUserMedia() mocking

[Manishearth]

It would be nice if Servo could request the backend to mock getUserMedia().

A simple enable_mediadevices_mocking() function that basically makes the mediadevices code proxy to https://github.com/servo/media/blob/121f943462b0b5e54fdd7a7546e1b8ffd19a87c9/servo-media/lib.rs#L32-L33 would be nice

Alternatively, we can do this entirely on the Servo side. I'm worried this would impact the fidelity of the tests: for example, we currently have a bug where one of the tests requests an impossible constraint which we pass down to gstreamer, and gstreamer asserts.

Overall making this still work with the constraint stuff might be tricky. It might just be a matter of testing against caps, though.

cc @jdm