Allow to send `browsingContext.downloadWillBegin` and `browsingContext.downloadEnd` for discarded browsing contexts

[whimboo]

Right now the BiDi spec for both the browsingContext.downloadWillBegin and browsingContext.downloadEnd events require a still open navigable for emitting the event. But tests could have already closed the page before any of the events (especially the ended one) will be emitted.

Currently the first step to Let navigation info be the result of "get the navigation info" given navigable and navigation status. is going to always fail, causing no event to be emitted.

Given that this is a valid use case we should consider enhancing both events to also being emitted when the navigable does not longer exist. Hereby we may have to update the BaseNavigationInfo type to allow the context to be null as well.