Added overdraw visualization.

``DrawCallVisualizer`` extends ``wtf.events.EventEmitter`` (so that ``OverdrawVisualizer`` can tell ``EventNavigatorToolbar`` when it is visible and the button style can be updated), which itself extends ``goog.Disposable``. Could I make Visualizer abstract and have it extend ``wtf.events.EventEmitter``?

Are there any closure annotations or style guide rules about abstract classes? I didn't see any online.