Handle cases where Safari doesn't dispatch the pagehide or visibilitychange events

[philipwalton]

On desktop Safari, when closing a tab by clicking the (X) on the tab UI itself, no visibilitychange or pagehide event is dispatched, so the page becomes terminated but the reported state never transitions away from active.

• https://bugs.webkit.org/show_bug.cgi?id=151234
• https://bugs.webkit.org/show_bug.cgi?id=151610

In order to at least provide a guarantee of hidden, we should listen for the beforeunload event and report a change to hidden if no event handlers set a returnValue or call preventDefault().

The following code achieves this:

addEventListener('beforeunload', (evt) => {
  setTimeout(() => {
    if (!(evt.defaultPrevented || evt.returnValue)) {
      // The state is now going to either FROZEN or TERMINATED.
    }
  }, 0);
});

But since adding an unconditional beforeunloaded handler will break the page navigation cache in Firefox, we should only add this logic in Safari.