Allow <video autoplay muted> in sanitizer

[Slayer95]

In the spirit of WICG/interventions#23, and to vanish GIF from the world.

[kpreid]

Nice idea. Unfortunately, a relationship between two attributes like that (allow autoplay only if muted) requires nontrivial custom code to implement (because JS could e.g. remove the muted attribute later). Still worth looking at but not until after we've cleaned up the code we'd right now be complicating to do this.