Fix example code in PageVisibility Notification demo

[kohenkatz]

The code in the PageVisibility tutorial's demo for showing a notification if the page is hidden is using the old Chrome notifications API instead of the new standard API. The example should be updated so that it works in current browsers. (The old code hasn't worked since around Chrome 22.)

I already have a working version in http://jsfiddle.net/kohenkatz/b2vteudu/, and I can make that into a pull request if that will be accepted.