Disallow dialog.requestFullscreen()

This way, interactions between the algorithms for fullscreen and
dialog are simplified. Example concern:
#102 (comment)

This also makes it easier to reinstate hierarchy restrictions:
#91

Tests: https://chromium-review.googlesource.com/c/chromium/src/+/684435

fullscreen.bs

@@ -246,6 +246,8 @@ these steps:
  <a href=https://www.w3.org/Math/draft-spec/chapter2.html#interf.toplevel>MathML <code>math</code></a>
  element. [[!SVG]] [[!MATHML]]
 
+ <li><p><var>pending</var> is not a <{dialog}> element.
+
  <li><p>The <a>fullscreen element ready check</a> for <var>pending</var> returns true.
 
  <li><p><a>Fullscreen is supported</a>.