Sandboxing and fullscreen

[annevk]

Apparently there's a rather large mess in HTML with respect to Fullscreen integration. See:

• Changes to allowfullscreen attribute should trigger parsing sandboxing directive as well whatwg/html#1240
• Algorithm of how fullscreen enabled flag is handled is not web-compatible whatwg/html#1481

To what extent should we try to preserve sandboxing integration (and also for the new allowusermedia feature) if that's not actually what implementations implement.

[annevk]

whatwg/html#1481 (comment) in particular has a suggestion for how to move forward.

[annevk]

Per whatwg/html#1481 (comment) we're just going with the simple buggy thing for now that was already shipped (since nobody really implements standards). Let's use a new issue if we actually want to give CSP control over fullscreen.