Bug 1383122 - P2. Amend mochitest to ensure null is properly converted to "null" string. r=jwwang

MozReview-Commit-ID: 13eeL3siXpy

--HG--
extra : rebase_source : 1cfa108414695ffc69c47d5fa9d494ee94c7b15a

dom/media/test/test_source_null.html

@@ -24,6 +24,9 @@ var v = document.getElementById('v');
 v.src = null; // crashes on NULL access if not handled
 
 ok(true, "setting video.src to null didn't crash!");
+var srcPath = v.src.split('/');
+ok(srcPath.length >= 3, "src should be within dom/media/test");
+is(srcPath[srcPath.length - 1], "null", "Setting src to null is handled like 'null' string");
 </script>
 </video>
 </body>