Allow a user to select from a set of video quality levels/resolutions/renditions/bitrates/variants/representations.

Was hoping to start this with a PR, but some research and discussion will be helpful first.

Related conversation: whatwg/html#562 from @dmlap

The proposed extension to VideoTrack seems promising.

partial interface VideoTrack {
  sequence<string> getAvailableRenditions();
  // promise resolves when change has taken effect
  Promise<void> setPreferredRendition(string rendition);
};

Something to solve for is "auto".

Ping @gkatsev @littlespex