Re: [whatwg/fullscreen] Remove top layer definitions, now that CSS Position 4 contains them. (PR #223)

@mfreed7 sounds good! Let's hammer out the details of point 2 then, what happens for a second call to `elm.requestFullscreen()` in different scenarios.

Bottom line first: no-op success when requesting fullscreen again matches current behavior. That's what this PR already does, so no changes required. 

The investigation:

**One scenario** is calling `requestFullscreen()` twice in the same click event handler:

```js
button.onclick = () => {
  const p1 = div.requestFullscreen();
  const p2 = div.requestFullscreen();
  // Does p2 reject and why?
}
```

This is tested in [element-request-fullscreen-twice.html](https://wpt.fyi/results/fullscreen/api/element-request-fullscreen-twice.html) and Chrome, Firefox and Safari all reject the second promise. Per spec (with https://github.com/whatwg/fullscreen/pull/153 just merged) it should reject because the user gesture was consumed by the first call.

**Another scenario** is calling `requestFullscreen()` on the element that's already fullscreen, in a second click event handler:

```js
button.onclick = () => {
  const p = div.requestFullscreen();
  // Does p reject if div is document.fullscreenElement?
}
```

This is tested in [element-request-fullscreen-same-element.html](https://wpt.fyi/results/fullscreen/api/element-request-fullscreen-same-element.html). That times out in Safari, I don't know why, so I tested this manually:

```html
button.onclick = () => {
    if (!document.fullscreenElement) {
        div.requestFullscreen().then(() => {
            console.log('1st request succeeded');
        }, (error) => {
            console.log('1st request failed:', error.message);
        });
    } else {
        div.requestFullscreen().then(() => {
            console.log('2nd request succeeded');
        }, (error) => {
            console.log('2nd request failed:', error.message);
        });
    }
}
```

I've found that Chrome, Firefox and Safari all resolve the 2nd promise, in other words it's a no-op.

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