Auto merge of #20837 - gterzian:tests_for_has_event_listener, r=cbrewster

Fix for document salvageable state

I noticed a bug where if the document was un-salavageable due to the presence of `beforeunload` listeners, this could be overwritten if there were no listeners for `unload`. This could also have happened if an iframe of the document had no listeners while the parent did.

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