The autoplay setting was binary and its default blocked all media,
including muted video, leaving sites that rely on muted autoplay
visibly broken. Replace it with a tri-state user-agent autoplay
policy (allow audio and video, block audio, or block audio and video)
defaulting to allowing only inaudible media to autoplay.
This is enforced through the media element's "allowed to play" check,
so unmuting a muted autoplay or calling `play()` cannot slip audio
past the policy; audible playback is permitted once the document has
been activated by the user. The policy lives in a dedicated
AutoplaySettings consulted from HTMLMediaElement instead of the
Permissions Policy "allowed to use feature" check it was previously
conflated with.
8dc8835b64