Reject exitFullscreen() in inactive documents

If the document becomes inactive after the request, then there will not
be another animation frame task, in which case the promise will be left
hanging instead.

Test: web-platform-tests/wpt#4350.

Fixes #67.

fullscreen.bs

@@ -314,8 +314,9 @@ attribute's getter must run these steps:
 <ol>
  <li><p>Let <var>promise</var> be a new promise.
 
- <li><p>If <var>doc</var>'s <a>fullscreen element</a> is null, reject <var>promise</var> with a
- <code>TypeError</code> exception, and return <var>promise</var>.
+ <li><p>If <var>doc</var> is not <a>fully active</a> or <var>doc</var>'s <a>fullscreen element</a>
+ is null, then reject <var>promise</var> with a <code>TypeError</code> exception and return
+ <var>promise</var>.
 
  <li><p>Let <var>resize</var> be false.
 

fullscreen.html

@@ -312,7 +312,7 @@ <h2 class="heading settled" data-level="3" id="api"><span class="secno">3. </spa
     <li>
      <p>Let <var>promise</var> be a new promise. </p>
     <li>
-     <p>If <var>doc</var>’s <a data-link-type="dfn" href="#fullscreen-element">fullscreen element</a> is null, reject <var>promise</var> with a <code>TypeError</code> exception, and return <var>promise</var>. </p>
+     <p>If <var>doc</var> is not <a data-link-type="dfn" href="https://html.spec.whatwg.org/multipage/browsers.html#fully-active">fully active</a> or <var>doc</var>’s <a data-link-type="dfn" href="#fullscreen-element">fullscreen element</a> is null, then reject <var>promise</var> with a <code>TypeError</code> exception and return <var>promise</var>. </p>
     <li>
      <p>Let <var>resize</var> be false. </p>
     <li>