Allow demo to work with prefixed & unprefixed MediaSource objects.

Change-Id: Iaba781e1c5d4158d154e7381596ccbc3244af98b
diff --git a/adaptive/dash-player.html b/adaptive/dash-player.html
index 07375bb..6f94191 100644
--- a/adaptive/dash-player.html
+++ b/adaptive/dash-player.html
@@ -22,6 +22,8 @@
 <script type="text/javascript">
 var mediaSourceObjectAvailable = false;
 try {
+  if (!window.MediaSource)
+    window.MediaSource = window.WebKitMediaSource;
   var testMediaSource = new MediaSource();
   includeScript('dash_player.js');
   mediaSourceObjectAvailable = true;