ui: Cache the output of View::OnPaint when the View isn't invalid.

cc/resources/drawing_display_item.h

Index: cc/resources/drawing_display_item.h
diff --git a/cc/resources/drawing_display_item.h b/cc/resources/drawing_display_item.h
index a3eef7711a0ad31cf4b7e7d129e3ec20dc1d3068..da302d87d9c82096443b1eef9470693ea86faf1d 100644
--- a/cc/resources/drawing_display_item.h
+++ b/cc/resources/drawing_display_item.h
@@ -33,6 +33,8 @@ class CC_EXPORT DrawingDisplayItem : public DisplayItem {
   size_t PictureMemoryUsage() const override;
   void AsValueInto(base::trace_event::TracedValue* array) const override;
 
+  scoped_ptr<DrawingDisplayItem> Clone();
+
  protected:
   explicit DrawingDisplayItem(skia::RefPtr<SkPicture> picture);