Popup forms: Save Page button does not work anymore

Quoting Oliver Lutzi:
"
I encountered the same problem (using a different extension). Basil Ell
and I figured out that the source of the bug is line 84 of the file
"SF_popupform.js".

You use the "setInterval()" method to call a function at specified
intervals and escape it once "window.readystate" returns "interactive".
As described in [1], "readystate" chages its return values from
"loading" to "interactive" and finally to "complete". If "readystate"
happens to change from state "loading" to "complete" between two calls,
the script never escapes "setInterval()" because the state "interactive"
was never observed.
"

Change-Id: Ic6f661bcdb3d67454c98989e8f96304131b9b61e