After bug 1415994 introduces LayerSession, LayerView will still have some miscellaneous stuff in it, 1) Overscroll handling code, which I think should move to LayerSession. 2) Assorted compositor APIs like draw listeners, pixel getters, and clear color setters. I think these APIs should go into their own helper class, e.g. "CompositorController". So adding a draw listener would become `mSession.getCompositorController().addDrawListener()`. 3) The `setSurfaceBackgroundColor` API. We don't have access to the SurfaceView in LayerSession anymore, so we need something else. We might be able to do similar thing in C++ and avoid Java altogether? Or we can let GeckoView handle it. 4) `handleToolbarAnimatorMessage` should be split between LayerSession and DynamicToolbarAnimator, depending on the action performed for a message.