[cssom-view] How to fulfill programmatic scroll promises?

[mustaqahmed]

This is a follow-up issue for #1562 where we already have the resolution that programmatic scroll methods in Element and Window would return Promises, and that for backward compatibility those Promises won't ever be rejected. Certain details of those Promises still need attention:

1. How will we fulfill the Promises to mark scroll completion vs interruption vs error? The PR for the issue above minimally covers this with Boolean values (true for completion and false otherwise). Perhaps an enum would be better?

2. Does one programmatic scroll request interrupt all pending promises? Or only the pending ones for the same scroller? The PR above assumes the latter. Is this the best choice?

3. Element.scrollIntoView() affects the ancestor scrollers of the element. How will a programmatic scroll request on any of those ancestors interrupt the first request? Synchronously at the time of the second request (no matter which of those ancestors are scrolling at the time of the second request)? Or asynchronously only if the both requests happen to be scrolling a common element at the same moment?