Pass a ui::PaintContext from ui::Layer to layer delegates.

ash/root_window_controller.cc

Index: ash/root_window_controller.cc
diff --git a/ash/root_window_controller.cc b/ash/root_window_controller.cc
index 83bb10826a9935a79f4215499924aa8a067dd543..1a56f5ecaa6e0f2603ee6ef2a073dbdae1150a14 100644
--- a/ash/root_window_controller.cc
+++ b/ash/root_window_controller.cc
@@ -235,7 +235,7 @@ class EmptyWindowDelegate : public aura::WindowDelegate {
   }
   bool CanFocus() override { return false; }
   void OnCaptureLost() override {}
-  void OnPaint(gfx::Canvas* canvas) override {}
+  void OnPaint(const ui::PaintContext& context) override {}
   void OnDeviceScaleFactorChanged(float device_scale_factor) override {}
   void OnWindowDestroying(aura::Window* window) override {}
   void OnWindowDestroyed(aura::Window* window) override { delete this; }