Bug 242930

Currently WebKit is the only rendering engine using the prefixed property for fullScreen: window.document.webkitFullscreenElement

the standard property is implemented on Gecko and Blink.
See https://developer.mozilla.org/en-US/docs/Web/API/Document/fullscreenElement#browser_compatibility

It is probably time to add aliases 

* fullscreenElement <- webkitFullscreenElement.
* requestFullscreen <- webkitRequestFullscreen
* exitFullscreen    <- webkitExitFullscreen


it is common to see code such as 

```
{
  key: "isFullscreen",
  value: function() {
      return window.document.fullscreenElement || window.document.webkitFullscreenElement || window.document.mozFullScreenElement || window.document.msFullscreenElement
  }
}
```

This can't be fully removed because of probable webcompat breakage.

worse it encourages code like this. (if you read this, do not do that)

```
const isFullScreenCapable =
    document.fullscreenElement || document.webkitFullScreenElement;
  const isWebkit = document.webkitFullScreenElement;

  if (isFullScreenCapable) {
    if (isWebkit) {
      document.webkitExitFullscreen();
    } else {
      document.exitFullscreen();
    }
  }

  if (isWebkit) {
    canvas.webkitRequestFullscreen();
  } else {
    canvas.requestFullscreen();
  }
```

and allow for fingerprinting.