Moving the source of a track

[alvestrand]

(Reopen consensus call closed on Sept 30 2021, which had 4 responses):

Dicsussion of use cases for MediaStreamTrack transfer in Google uncovered several use cases where the restriction of a source to one context was severely harmful to the use case.

The obvious case was when one moved a track to display in a different tab, and then closed the original tab.

In this case, it would be better to define the privacy indicators as an UA-level property, and the source as being independent of context.