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

ui/compositor/layer_delegate.h

Index: ui/compositor/layer_delegate.h
diff --git a/ui/compositor/layer_delegate.h b/ui/compositor/layer_delegate.h
index 9312e02af91308e02768b028c61ab2811592b1d0..c3c4e39aed6ede950c787357720ea90b85bc5e9c 100644
--- a/ui/compositor/layer_delegate.h
+++ b/ui/compositor/layer_delegate.h
@@ -14,13 +14,13 @@ class Rect;
 }
 
 namespace ui {
+class PaintContext;
 
 // A delegate interface implemented by an object that renders to a Layer.
 class COMPOSITOR_EXPORT LayerDelegate {
  public:
-  // Paint content for the layer to the specified canvas. It has already been
-  // clipped to the Layer's invalid rect.
-  virtual void OnPaintLayer(gfx::Canvas* canvas) = 0;
+  // Paint content for the layer to the specified context.
+  virtual void OnPaintLayer(const PaintContext& context) = 0;
 
   // A notification that this layer has had a delegated frame swap and
   // will be repainted.