[XHR2] progress events dispatch rate

Currently XMLHttpRequest Level 2 suggests 350ms +- 200ms as the interval  
for progress events. This was simply copied from what was specified for  
<video>. It has been pointed out that for XMLHttpRequest the use cases are  
a bit different so that 50ms or 100ms might be more desirable. (With 50ms  
you'd get 20fps ideally.)

I think I'll change 350ms +- 200ms to simply 50ms as the recommended  
interval (or for every byte, whichever is _least_ frequent, blah blah).  
Anyone who'd like something else?


-- 
Anne van Kesteren
<http://annevankesteren.nl/>
<http://www.opera.com/>