unload event

[styfle]

https://groups.google.com/a/chromium.org/g/blink-dev/c/dvusqw9-IhI

A Permission-Policy for creating unload event listeners will be added.

Initially, the default policy will be set to allow. From there, Chrome will gradually migrate the default policy to deny (i.e. increasingly disallow the creation of unload event listeners, eventually reaching a state where deny fully becomes the default policy). The ultimate goal is to remove support for unload event.