Add experimental function to log events using the sendBeacon API

https://www.mediawiki.org/wiki/Extension:EventLogging/sendBeacon
https://etherpad.wikimedia.org/p/sendBeacon-2014-10-03

Bug: 42815
Change-Id: I022b13cfd74a691e116348b7bfb8c6f8698b2b41
Merge "Add experimental function to log events using the sendBeacon API"