In VitalSource custom/synthetic "unload" events are dispatched at the book's
container frame when switching chapters. Make sure these don't trigger the code
in the `PortRPC` class that is meant to be called when the window is unloaded.

These synthetic events were triggered after every chapter navigation. In Safari
<= 15 the second and subsequent events would cause an error in the
`currentWindow.parent.postMessage` call because the port had already been
transferred after the first event was handled.