Bug 1282038 - Follow-up: Ensure unprefixed fullscreen API is enabled. r=me

--HG--
extra : rebase_source : 839c307c4842da7d42ce68e55fb26d85c905980c

dom/base/test/test_youtube_flash_embed.html

@@ -20,9 +20,13 @@
        self[msg.fn].apply(null, msg.args);
      }
      function onLoad() {
-       // The test file must be loaded into youtube.com domain
-       // because it needs unprivileged access to fullscreenEnabled.
-       ifr.src = "https://mochitest.youtube.com" + path;
+       SpecialPowers.pushPrefEnv({
+         "set": [["full-screen-api.unprefix.enabled", true]]
+       }, function() {
+         // The test file must be loaded into youtube.com domain
+         // because it needs unprivileged access to fullscreenEnabled.
+         ifr.src = "https://mochitest.youtube.com" + path;
+       });
      }
     </script>
   </head>