Currently, the mechanism for transferring an ImageBitmap object uses RefPtr<StaticBitmapImage> as a vehicle for passing the pixel data between realms.  This does not work if the target realm is in another process.

A cross-process transfer can arise with the following use case:
1. do a window.open()
2. Navigating the new window via omnibox to a different site will put it in a different process
3. Doing a postMessage to its window handle will result in a cross-process postMessage.

out-of-process iframe are another problematic use case.