Register visibility-change early

[tunetheweb]

Fixes #502

@philipwalton I did what you suggested in #502 (comment) but I've a couple of observations/questions:

Code Location
As the visibility-change event handlers are registered and unregistered after first page hide, we don't actually end up using any of the existing getVisibilityWatcher code. Additionally the only two functions that need this (onCLS and onINP) don't even import that at present. So not sure the code really lives best in here, or if it should just be in it's own ./lib/onHidden.js file? Then again, it might make getVisibilityWatcher more accurately name by putting it in this file (otherwise it maybe really should be called getInitHiddenTime)!

Update: Agreed it would go here.

Test case
Should we add a test case for this? Ideally yes of course, and it's possible to set up the test case similar to those reported (for example: old and new), but it would be a fair bit of extra code to add to the test suite, potentially complicating the existing test cases. And we don't have test cases for other such issues (though probably should, and maybe more so in this case since it's affected at least one user). Can you see an easy way to add a test case for this? Maybe as a unit test?

Update: Added a test case.

[tunetheweb]

I got bored of manually testing this each time so added a test case. Despite my worries in the original comment it wasn't too bad to add.