Use sanitizeFor *and* setHTML

[mozfreddyb]

Daniel was looking at the usercounters for the API and the (very low) numbers are in favor of sanitizeFor. I'd like to argue that people use pages like this one to test & learn about the API and that is likely skewing the numbers.

I'll send a tiny PR that makes sure the page uses both if that's OK with y'all :)

[mikewest]

I don't think Chrome's UseCounter numbers at the moment will tell you anything useful: we haven't shipped the API, so the set of users poking at it is tiny and entirely non-representative.

That said, if you'd like to add setHTML to the playground, go for it! :)