IndexedDB: Result of failed request should be |undefined|

LayoutTests/storage/indexeddb/mozilla/add-twice-failure-expected.txt

 Test IndexedDB behavior adding the same property twice
 
 On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
 
 
 indexedDB = self.indexedDB || self.webkitIndexedDB || self.mozIndexedDB || self.msIndexedDB || self.OIndexedDB;
 
 dbname = "add-twice-failure.html"
 indexedDB.deleteDatabase(dbname)
 indexedDB.open(dbname)
 objectStore = db.createObjectStore('foo');
 key = 10
 request = objectStore.add({}, key);
 PASS request.result is key
 request = objectStore.add({}, key);
+PASS event.target.result is undefined.
+PASS event.target.error is non-null.
 PASS event.target.error.name is 'ConstraintError'
 PASS successfullyParsed is true
 
 TEST COMPLETE