Bug 1305336. Part 5 - use manifestVideo() so pref changes can take effect as expected. r=pehrsons

MozReview-Commit-ID: 2YXn4jSfivh

--HG--
extra : rebase_source : af98c0e8291caaae41e1a4e4dcd7b1b3b3b5654a

dom/media/test/manifest.js

@@ -1469,12 +1469,12 @@ function getPlayableVideo(candidates) {
 }
 
 function getPlayableVideos(candidates) {
-  var v = document.createElement("video");
+  var v = manifestVideo();
   return candidates.filter(function(x){return /^video/.test(x.type) && v.canPlayType(x.type);});
 }
 
 function getPlayableAudio(candidates) {
-  var v = document.createElement("audio");
+  var v = manifestVideo();
   var resources = candidates.filter(function(x){return /^audio/.test(x.type) && v.canPlayType(x.type);});
   if (resources.length > 0)
     return resources[0];