Fix typo in add_tracing_toggle.
Fix typo in add_tracing_toggle.

SketchUp/testup-2,thomthom/testup-2,SketchUp/testup-2,thomthom/testup-2,SketchUp/testup-2,SketchUp/testup-2,SketchUp/testup-2,thomthom/testup-2,thomthom/testup-2,thomthom/testup-2,thomthom/testup-2,SketchUp/testup-2