Add an intermediate function to generate 2 paint-order lists.

Source/core/testing/Internals.idl

Index: Source/core/testing/Internals.idl
diff --git a/Source/core/testing/Internals.idl b/Source/core/testing/Internals.idl
index 60c359afc35577ad17b63c676ab850c52683078d..0fa77f7b005cbab9d89f527f026e57ce7c28b08d 100644
--- a/Source/core/testing/Internals.idl
+++ b/Source/core/testing/Internals.idl
@@ -172,6 +172,9 @@
     const unsigned short LAYER_TREE_INCLUDES_PAINTING_PHASES = 8;
     [RaisesException] DOMString layerTreeAsText(Document document, optional unsigned short flags);
 
+    [RaisesException] NodeList paintOrderListBeforePromote(Element element);
+    [RaisesException] NodeList paintOrderListAfterPromote(Element element);
+
     [RaisesException] DOMString scrollingStateTreeAsText(Document document);
     [RaisesException] DOMString mainThreadScrollingReasons(Document document);
     [RaisesException] ClientRectList nonFastScrollableRects(Document document);