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

ui/aura/demo/demo_main.cc

Index: ui/aura/demo/demo_main.cc
diff --git a/ui/aura/demo/demo_main.cc b/ui/aura/demo/demo_main.cc
index 2e4ac3be1f946d9df184d401e6d647829abb180d..bf23d451f9892b5376b671ed398e54b65b4cec22 100644
--- a/ui/aura/demo/demo_main.cc
+++ b/ui/aura/demo/demo_main.cc
@@ -19,6 +19,7 @@
 #include "ui/aura/window_delegate.h"
 #include "ui/aura/window_tree_host.h"
 #include "ui/base/hit_test.h"
+#include "ui/compositor/paint_context.h"
 #include "ui/compositor/test/in_process_context_factory.h"
 #include "ui/events/event.h"
 #include "ui/gfx/canvas.h"
@@ -62,7 +63,8 @@ class DemoWindowDelegate : public aura::WindowDelegate {
   }
   bool CanFocus() override { return true; }
   void OnCaptureLost() override {}
-  void OnPaint(gfx::Canvas* canvas) override {
+  void OnPaint(const ui::PaintContext& context) override {
+    gfx::Canvas* canvas = context.canvas();
     canvas->DrawColor(color_, SkXfermode::kSrc_Mode);
     gfx::Rect r;
     canvas->GetClipBounds(&r);