Intent to Ship: Autoplay Policy Detection API

[alast...@gmail.com]

I intend to turn on the Autoplay Policy Detection API by default on all platforms on Fx112. It has been developed behind the `dom.media.autoplay-policy-detection.enabled` preference.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1812189

Proposed Standard: https://w3c.github.io/autoplay/

Proposal Repository: https://github.com/w3c/autoplay

MDN: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/getAutoplayPolicy

Platform coverage: all platform, pref on by default

DevTools bug: None

Other browsers:
None, Firefox is the first browser to support that API, see following link for more information.
https://github.com/w3c/autoplay#stakeholder-feedback--opposition

Testing: https://wpt.fyi/results/autoplay-policy-detection?label=experimental&label=master&aligned

Use cases:
Most user agents have their own mechanisms to block autoplay media, and those mechanisms are implementation-specific. Web developers need to have a way to detect if autoplaying media is allowed or not in order to make actions, such as selecting alternate content or improving the user experience while media is not allowed to autoplay. For instance, if a user agent only blocks audible autoplay, then web developers can replace audible media with inaudible media to keep media playing, instead of showing a blocked media which looks like a still image to users. If the user agent does not allow any autoplay media, then web developers could stop loading media resources and related tasks to save the bandwidth and CPU usage for users.

Currently, this specification only handles HTMLMediaElement (video and audio) and Web Audio API. This specification does not handle Web Speech API and animated image (GIF animation).