Intent to implement and ship: MediaStreamTrack's .getConstraints() and .getSettings()

[Jan-Ivar Bruaroey]

Summary:

Let developers read back what constraints have been put on
MediaStreamTracks by navigator.mediaDevices.getUserMedia() or
track.applyConstraints(), as well as the resulting net settings. The new
methods for this are track.getConstraints() and track.getSettings()
respectively. The settings available are limited to those we currently
have constraints for.

Motivation:

For example, this can be used to learn the dimensions of a camera
stream's video track, without having to create a <video> element, assign
that element's srcObject to the stream, and wait for metadata to load on
the element.

Bugs:
https://bugzil.la/1213517
https://bugzil.la/1286096
https://bugzil.la/1088621

Link to standard:
https://w3c.github.io/mediacapture-main/getusermedia.html#media-stream-track-interface-definition

Platform coverage: all

Estimated or target release: Firefox 50

Preference behind which this will be implemented: no pref.