Add support for upcoming promise behavior

[stipsan]

Smooth scrolling will return promises in the future: w3c/csswg-drafts#1562

Suggested implementation here is:
If the browser is returning a promise, pass it through and let the browser decide how it's resolved.
If the browser isn't returning a promise, but the behavior is set to "smooth", use Promise.reject as there is no way of knowing when scrolling is complete anyway so userland code needs to handle both cases.