The current version of `isSourceWindow` raised an error for (1) older
browser that doesn't support ServiceWorker and (2) web pages served from
non-secured http connections.

I tested this alternative fix but it didn't work for me in the latest
version of Chrome:
https://stackoverflow.com/questions/6229301/a-clean-way-of-checking-whether-an-object-is-an-instance-of-window-constructor/6229603#6229603