Re: [whatwg/dom] Add a fully active check for EventTarget in event listener inner invoke (PR #1085)

> Firefox has checks in the webidl callbacks layer to prevent calling listener in the globals which are "gone".

The global of the `EventTarget` or the listener? In this case, the checks are about the global of the `EventTarget`, i.e. the listener's global can still be attached, but the target's is not.

When I filed the issue, I did some debugging in FF code to see if I could figure out where the check was happening. I observed the following:

1. If the `EventTarget`'s global is detached when dispatching events, we hit an early out in [EventTargetChainItem::HandleEvent](https://searchfox.org/mozilla-central/source/dom/events/EventDispatcher.cpp#318)() because the `EventListenerManager` is null.
2. If the `EventTarget`'s global is detached during dispatch, I found iteration during `HandleEventInternal()` abruptly ends because the listeners got cleared. This happens, for example, if there are two event listeners and the first detaches the event target's global. I found [code](https://searchfox.org/mozilla-central/source/dom/base/nsIGlobalObject.cpp#65) that "disconnects" the event targets and clears listeners which I *think* caused this, but regardless I definitely observed iteration ending after the first event listener detached the event target's global.

I don't know this code nearly as well as y'all, but this is what I observed with some hacky printf debugging; hopefully that's helpful.

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