Fix a followup notification paging error with partial objects

Summary: Ref T13623. In D21603, I made the "partial object" this query returns a raw row, which paging keys can no longer be extracted from properly.

Test Plan: Paged notifications to page 2, no longer saw an error.

Maniphest Tasks: T13623

Differential Revision: https://secure.phabricator.com/D21609

src/applications/notification/query/PhabricatorNotificationQuery.php

@@ -181,7 +181,7 @@ final class PhabricatorNotificationQuery
 
   protected function newPagingMapFromPartialObject($object) {
     return array(
-      'key' => $object->getChronologicalKey(),
+      'key' => $object['chronologicalKey'],
     );
   }