iFrame Communicator - Add form loaded action

[coppercup]

It would be very helpful if the Accept Hosted payment form iFrame Communicator could send a communication indicating when the payment form is fully loaded in the iFrame.

We are seeing issues in some browsers (especially Safari) where scrolling while the form is still loading (before the iFrame has been resized) can result in portions of the form being hidden and no scroll bars to bring it back.

If we had notification of the form being fully loaded, we could block pointer events and scrolling for the iframe until after the the page is fully loaded.