Implement MediaStreamTrack.getConstraints

This stores the currently valid constraints inside a
MediaStreamTrack object, in the form of a WebMediaConstraints
variable, and returns it with the GetConstraints operation.

http://w3c.github.io/mediacapture-main/getusermedia.html#widl-MediaStreamTrack-getConstraints-MediaTrackConstraints

BUG=543997

Review URL: https://codereview.chromium.org/1910463002

Cr-Commit-Position: refs/heads/master@{#390074}
9 files changed