Let Chromium manage document.visibilityState and document.hidden

Chromium already includes the necessary plumbing to manage the
visibility properties and `visibilitychange` event so this gets rid of
most of our custom logic for `BrowserWindow` and `BrowserView`.

Note that `webview` remains unchanged and is still affected by the issues
listed below.

User facing changes:

- The `document` visibility properties and `visibilitychange` event are
  now also updated/fired in response to occlusion changes on macOS. In
  other words, `document.visibilityState` will now be `hidden` on macOS
  if the window is occluded by another window.

- Previously, `visibilitychange` was also fired by *both* Electron and
  Chromium in some cases (e.g. when hiding the window). Now it is only
  fired by Chromium so you no longer get duplicate events.

- The visiblity state of `BrowserWindow`s created with `{ show: false }`
  is now initially `visible` until the window is shown and hidden.

- The visibility state of `BrowserWindow`s with `backgroundThrottling`
  disabled is now permanently `visible`.

This should also fix #6860 (but not for `webview`).

lib/browser/guest-view-manager.js

@@ -281,7 +281,7 @@ const watchEmbedder = function (embedder) {
     for (const guestInstanceId of Object.keys(guestInstances)) {
       const guestInstance = guestInstances[guestInstanceId]
       if (guestInstance.embedder === embedder) {
-        guestInstance.guest.send('ELECTRON_RENDERER_WINDOW_VISIBILITY_CHANGE', visibilityState)
+        guestInstance.guest.send('ELECTRON_GUEST_INSTANCE_VISIBILITY_CHANGE', visibilityState)
       }
     }
   }