Send the serialized string to the track_function in split_test_module.

common/lib/xmodule/xmodule/split_test_module.py

@@ -226,7 +226,7 @@ class SplitTestModule(SplitTestFields, XModule):
         Record in the tracking logs which child was rendered
         """
         # TODO: use publish instead, when publish is wired to the tracking logs
-        self.system.track_function('xblock.split_test.child_render', {'child-id': self.child.scope_ids.usage_id})
+        self.system.track_function('xblock.split_test.child_render', {'child-id': self.child.scope_ids.usage_id.to_deprecated_string()})
         return Response()
 
     def get_icon_class(self):