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/chromium-spec.js

@@ -48,35 +48,6 @@ describe('chromium feature', function () {
     })
   })
 
-  describe('document.hidden', function () {
-    var url = 'file://' + fixtures + '/pages/document-hidden.html'
-
-    it('is set correctly when window is not shown', function (done) {
-      w = new BrowserWindow({
-        show: false
-      })
-      w.webContents.once('ipc-message', function (event, args) {
-        assert.deepEqual(args, ['hidden', true])
-        done()
-      })
-      w.loadURL(url)
-    })
-
-    it('is set correctly when window is inactive', function (done) {
-      if (isCI && process.platform === 'win32') return done()
-
-      w = new BrowserWindow({
-        show: false
-      })
-      w.webContents.once('ipc-message', function (event, args) {
-        assert.deepEqual(args, ['hidden', false])
-        done()
-      })
-      w.showInactive()
-      w.loadURL(url)
-    })
-  })
-
   xdescribe('navigator.webkitGetUserMedia', function () {
     it('calls its callbacks', function (done) {
       navigator.webkitGetUserMedia({