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`).

spec/webview-spec.js

@@ -485,9 +485,7 @@ describe('<webview> tag', function () {
             typeofArrayPush: 'number',
             typeofFunctionApply: 'boolean',
             typeofPreloadExecuteJavaScriptProperty: 'number',
-            typeofOpenedWindow: 'object',
-            documentHidden: isCI,
-            documentVisibilityState: isCI ? 'hidden' : 'visible'
+            typeofOpenedWindow: 'object'
           }
         })
         done()