ui: Introduce CanvasPainter, remove PaintContext(gfx::Canvas*).

ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc

Index: ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
diff --git a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
index a927a48e8b08123b740bd7fc3b70c426bdbef37a..a663f3894b9873494995848fd43dfd6fd9ecd252 100644
--- a/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
+++ b/ui/views/widget/desktop_aura/desktop_window_tree_host_win.cc
@@ -678,10 +678,6 @@ void DesktopWindowTreeHostWin::ResetWindowControls() {
   GetWidget()->non_client_view()->ResetWindowControls();
 }
 
-void DesktopWindowTreeHostWin::PaintLayeredWindow(gfx::Canvas* canvas) {
-  GetWidget()->GetRootView()->Paint(ui::PaintContext(canvas));
-}
-
 gfx::NativeViewAccessible DesktopWindowTreeHostWin::GetNativeViewAccessible() {
   return GetWidget()->GetRootView()->GetNativeViewAccessible();
 }