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

cc/resources/drawing_display_item.cc

Index: cc/resources/drawing_display_item.cc
diff --git a/cc/resources/drawing_display_item.cc b/cc/resources/drawing_display_item.cc
index 91ab3fb0d38103b37a08a0ab9e8a9c25df79ee49..a37729f7bd4542bcaf197b67e18541ae06d18de7 100644
--- a/cc/resources/drawing_display_item.cc
+++ b/cc/resources/drawing_display_item.cc
@@ -62,4 +62,8 @@ void DrawingDisplayItem::AsValueInto(
   array->EndDictionary();
 }
 
+scoped_ptr<DrawingDisplayItem> DrawingDisplayItem::Clone() {
+  return Create(picture_);
+}
+
 }  // namespace cc