Fix fullscreen polyfill test.

Only install a getter for fullscreenElement if the property does not
exist.  Previously, we installed the getter if the property existed
but was null.

Change-Id: Ibea5bc63a75a98cd1d50c56ca99fac2aa38e449b

lib/polyfill/fullscreen.js

@@ -49,7 +49,7 @@ shaka.polyfill.Fullscreen.install = function() {
                          proto.msExitFullscreen ||
                          proto.webkitExitFullscreen;
 
-  if (!document.fullscreenElement) {
+  if (!('fullscreenElement' in document)) {
     Object.defineProperty(document, 'fullscreenElement', {
       get: function() {
         return document.mozFullScreenElement ||