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

ui/compositor/paint_context.h

Index: ui/compositor/paint_context.h
diff --git a/ui/compositor/paint_context.h b/ui/compositor/paint_context.h
index e091c5efbcba9ef130ac6ac8111fd0a320b519ed..79c91424e2b232086a03743e1b558312f899cc6e 100644
--- a/ui/compositor/paint_context.h
+++ b/ui/compositor/paint_context.h
@@ -38,10 +38,6 @@ class COMPOSITOR_EXPORT PaintContext {
                const gfx::Rect& bounds,
                const gfx::Rect& invalidation);
 
-  // Construct a PaintContext that will re-paint everything (no consideration
-  // for invalidation).
-  explicit PaintContext(gfx::Canvas* canvas);
-
   // Clone a PaintContext with an additional |offset|.
   PaintContext(const PaintContext& other, const gfx::Vector2d& offset);