IndexedDB: Stop using webkit-prefixed versions of APIs in tests

content/test/data/indexeddb/database_callbacks_first.html

 <script>
 
     function gcAndDetach() {
       // Collect the IDBRequest so that only the indexeddb message dispatcher
       // has a reference to IDBDatabase.
       gc();
       // Make Document::detach run stopActiveDOMObjects.
       location.href = 'database_callbacks_second.html';
     }
     function openConnection() {
-      var idbRequest = webkitIndexedDB.open("database_callbacks_first");
+      var idbRequest = indexedDB.open("database_callbacks_first");
       // setTimeout is needed so that the IDBRequest returned by
-      // webkitIndexedDB.open() can be garbage collected.
+      // indexedDB.open() can be garbage collected.
       idbRequest.onsuccess = function() { setTimeout(gcAndDetach, 0) };
     }
     openConnection();
 </script>