[whatwg] setTimeout clamps

I've been looking at the setTimeout clamping in Gecko recently, and one 
thing I've been interesting in trying is to clamp differently in 
different windows (for example, have a lower clamp in active windows and 
a higher one in inactive ones).  That's not something the spec seems to 
allow right now, but I think it would be beneficial to users in terms of 
preventing inactive windows from accidentally hogging the CPU.

Note that right now in Gecko "active window" means "currently selected 
tab in some browser window" and "inactive window" means "background 
tab", but that's subject to change.

-Boris