Add VideoPlaybackQuality and HTMLVideoElement»getVideoPlaybackQuality()

[sideshowbarker]

Move the specifications for the VideoPlaybackQuality interface and the HTMLVideoElement»getVideoPlaybackQuality() method from the Media Playback Quality specification at https://w3c.github.io/media-playback-quality/ into the HTML standard.

That interface and method:

• are implemented in all current engines
• have some tests at https://wpt.fyi/results/media-playback-quality (minimal ones, just for the IDL conformance)
• are documented in MDN at https://developer.mozilla.org/en-US/docs/Web/API/VideoPlaybackQuality and https://developer.mozilla.org/en-US/docs/Web/API/HTMLVideoElement/getVideoPlaybackQuality

Related: https://github.com/w3c/whatwg-coord/issues/8

[Ahmad-S792]

@sideshowbarker - just to flag, I just fixed failing 'idlharness' tests in Safari / WebKit and now on trunk, it passes all.

[annevk]

cc @whatwg/media