Currently, some extensions/ code assumes workers from extensions always come from '/' scope, e.g.:
https://cs.chromium.org/chromium/src/extensions/browser/event_listener_map.cc?rcl=688bd939e3ccbf9ad9d00326b55f52f2b5fce62a&l=249

These would be wrong for "lazy" events that are persisted into prefs.

For event information persisted into ExtensionPrefs, we need to remember worker's scope, so that the correct worker can be started when dispatching its event.