Write sentences--initial caps, periods, etc. Some more exposition might be useful, for example, whether f is called during onload or not could be briefly spelled out.

You don't actually seem to use testInIFrame with non-empty URL in these tests, so why not remove it for now. You can elaborate this function later if you need it.

I also think this "context" is excessively elaborate. Can't you just call iframe.remove() in the finally block and be done with it? Why do you need a separate object, prototype, etc.