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

ui/aura/test/test_window_delegate.h

Index: ui/aura/test/test_window_delegate.h
diff --git a/ui/aura/test/test_window_delegate.h b/ui/aura/test/test_window_delegate.h
index 1f1b722d7282356784c60b9b48943df3ec3d3ab1..fd4f5f88cba770cc3cef16428d20dd7ccdd60707 100644
--- a/ui/aura/test/test_window_delegate.h
+++ b/ui/aura/test/test_window_delegate.h
@@ -55,7 +55,7 @@ class TestWindowDelegate : public WindowDelegate {
       const gfx::Point& location) override;
   bool CanFocus() override;
   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(Window* window) override;
   void OnWindowDestroyed(Window* window) override;
@@ -86,7 +86,7 @@ class ColorTestWindowDelegate : public TestWindowDelegate {
   // Overridden from TestWindowDelegate:
   void OnKeyEvent(ui::KeyEvent* event) override;
   void OnWindowDestroyed(Window* window) override;
-  void OnPaint(gfx::Canvas* canvas) override;
+  void OnPaint(const ui::PaintContext& context) override;
 
  private:
   SkColor color_;