Allow pages with beforeunload listener to enter BFCache for desktop

[Sean Feng]

Hi all,

In Bug 1837045, I am going to enable the docshell.shistory.bfcache.ship_allow_beforeunload_listeners pref in Release so that pages with beforeunload listener can be BFCached. This is for desktop only. 

Firefox is the only browser that prevents pages with beforeunload listener from entering BFCache on Desktop. We have had this pref enabled for EARLY_BETA for a long time, and we think it's good to enable it in Release now.

Thanks,

Sean