applyConstraints() returns a promise.

• If the track is "ended", the promise is resolved.
• Otherwise, the promise is either...
  • ...rejected if there is a problem.
  • ...resolved if no problem is detected.

While this is somewhat useful, it might be more useful to specify that the promise is only resolved after the new set of constraints has taken effect, and all new frames emitted by the track are guaranteed to be subject to the new set of constraints.