Bug 205942

** Current behavior: **

When opening the tab switcher in Safari on iOS, the `visibilitychange` event will only fire once a user switches to a new tab. If the user instead closes the tab, then no `visibilitychange` event is fired (neither is `pagehide`, `beforeunload`, or `unload` FWIW, though I don't think those events should offer the same guarantees as `visibilitychange`).

STR:

1. Visit https://visibilitychange-logger.glitch.me on an iPhone
2. Click the tab switcher button in the bottom right
3. Close the tab
4. Re-visit https://visibilitychange-logger.glitch.me and notice that no events were logged

(note: `visibilitychange` events *are* logged if you actually switch to another tab)

** Proposed behavior: **

In my opinion, the `visibilitychange` event should fire as soon as the tab switcher button is pressed. That would guarantee that at least the hidden state is triggered regardless of whether the user switches tabs or closes the tab. (This is also consistent with how the `visibilitychange` event fires on Chrome on both Android and iOS)

Alternatively, if https://bugs.webkit.org/show_bug.cgi?id=151234 can be fixed to ensure the `visibilitychange` event fires any time the page goes away (including closing it from the tab switcher), that would also solve this problem.