disable page unload beacon about boomerang

[bluesmoon]

@mems boomerang doesn't attach to the unload event except for really old browsers that don't have bfcache. The boomerang unload event is the browser's beforeunload or pagehide event.

[mems]

Also note that:

The most important way to optimize for bfcache in all browsers is to never use the unload event. Ever!

[...]

Instead of using the unload event, use the pagehide event. The pagehide event fires in all cases where the unload event currently fires, and it also fires when a page is put in the bfcache.

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