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

ui/aura/window_delegate.h

Index: ui/aura/window_delegate.h
diff --git a/ui/aura/window_delegate.h b/ui/aura/window_delegate.h
index 1927ccd4adb11f51e9175903d6ef839082503331..c344ac23bf3759ce0507c30fe27eeae87c379146 100644
--- a/ui/aura/window_delegate.h
+++ b/ui/aura/window_delegate.h
@@ -25,6 +25,7 @@ class GestureEvent;
 class KeyEvent;
 class Layer;
 class MouseEvent;
+class PaintContext;
 class TextInputClient;
 class Texture;
 class TouchEvent;
@@ -69,8 +70,8 @@ class AURA_EXPORT WindowDelegate : public ui::EventHandler {
   // Invoked when mouse capture is lost on the window.
   virtual void OnCaptureLost() = 0;
 
-  // Asks the delegate to paint window contents into the supplied canvas.
-  virtual void OnPaint(gfx::Canvas* canvas) = 0;
+  // Asks the delegate to paint window contents into the supplied context.
+  virtual void OnPaint(const ui::PaintContext& context) = 0;
 
   // Called when the window's device scale factor has changed.
   virtual void OnDeviceScaleFactorChanged(float device_scale_factor) = 0;