add beacon to find out whether localStorage and sessionStorage work on page loads

we want to know so we can tell if it's worth to add personalisation settings into there

we are going to add a test for indexedDB as well in another PR
there isn't anything wrong with this, but if you defined this map in such a way that `BrowserStorageStatsSwitch` was tested, it would be really easy to find and pull this out after switch expiry (people don't always check git history).