define permission persistence

fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=22214
built from https://github.com/fluffy/webrtc-w3c/pull/33
As I understood it, the resolution about "taking away labels when all devices have been closed", refers to the MediaDeviceInfo array you get from MediaDevices.enumerateDevices() (called after all devices have been closed), not MediaStreamTrack objects. The approach so far has been to leave the informational properties on MediaStreamTrack as is even though it's ended.