Add Picture in Picture to unloading document cleanup steps

[chrisn]

See #8906.

One question I have is what order should these steps be in. For now, I've added the PiP step after WebTransport.

This PR depends on w3c/picture-in-picture#260

@marcoscaceres @theIDinside

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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💥 Error: Wattsi server error 💥

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Parsing MDN data...
Parsing...
Parse Error: (113966,96) parse error while closing p element

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