Delete generated jsonp-callbacks

[markelog]

Now we assign it to undefined when undefined when function is executed, we did this because we can't properly delete properties from the window objects on oldIE.

This issue still exist for supported IE8, but i think we should discuss using removeProp method in both branches instead.

[dbkaplun]

While deleteing the function makes more semantic sense, V8 will deoptimize any objects that have had properties deleted. Just something to keep in mind.

[markelog]

This is pure guessing, first of all, window object should be already deoptimized, since there is a lot of bounded properties, second, window is special host object so there might be another type of optimizations in play.

I wouldn't rely on those assumptions, until they proved by the performance benchmarking.

[markelog]

@timmywil, @dmethvin, @jaubourg need your opinions here.

Also, would like to know if our perception of the #2317 didn't change.

[dmethvin]

I agree that window is pretty magic and global so I doubt optimization comes into play here.

Would it make sense to create a single object on window at initialization and put all our JSONP callbacks there? That would work on IE8 too since we'd be deleteing the properties out of that regular object rather than on the global.

[markelog]

Which would mean we would expose three globals and as soon as IE8 dead we could eliminate it, cool.
Although, it seems we would need to do it only because of the one browser.

[timmywil]

I like the idea of using removeProp, which means we can delete in every browser except IE8 and IE8 receives no change.