Bug 1083729 - Avoid callscreen window leakage r=etienne_s

Because of a platform bug, the callscreen window gets leaked in bfcache,
as documented on bug 1078448 and stated in comment 8. Adding an empty
'unload' event listener allows us to avoid the leakage.
Merge pull request #25217 from lissyx/bug1083729

Bug 1083729 - Avoid callscreen window leakage r=etienne_s