title: requestFullscreen is not returning a promise
body: <p>I'm facing an unexpected behavior and wanted to be sure i'm not missing something before i'm filling a bug.<br>
  I've played with <code>requestFullscreen</code> on <code>chrome</code> and <code>firefox</code> and noticed it's not returning a promise like stated in the <a href="https://fullscreen.spec.whatwg.org/#api" rel="nofollow noreferrer">spec</a>.  </p>
  
  <p>Example for <code>webkit</code>:  </p>
  
  <pre><code>el.addEventListener('click', () =&gt; {
      const p = el.webkitRequestFullScreen();
      p.then(() =&gt; {console.log('full screen')});
  });
  </code></pre>
  
  <p>I'm getting: </p>
  
  <blockquote>
    <p>Uncaught TypeError: Cannot read property 'then' of undefined
        at HTMLDivElement.el.addEventListener</p>
  </blockquote>
  
  <p>Same goes for <code>firefox</code>:  </p>
  
  <pre><code>el.addEventListener('click', () =&gt; {
      const p = el.mozRequestFullScreen();
      p.then(() =&gt; {console.log('full screen')});
  });
  </code></pre>
  
  <blockquote>
    <p>TypeError: p is undefined</p>
  </blockquote>
  
  <p>Am i reading the spec wrong? shouldn't i expect that promise? </p>