Currently, when accelerated compositing is on, scheduling is done by the render widget if !USE(THREADED_COMPOSITING), and by the mainthreadproxy if USE(THREADED_COMPOSITING) and !CCSettings.enableThread. To remove this mess, we need to implement some sort of renderer scheduling algorithm on the CCLayerTreeHostImplProxy so that we can end-of-life the render_widget's compositing scheduler.