Introduce ComposedTreeTraversal::previousPostOrder()

Source/core/dom/shadow/ComposedTreeTraversal.h

Index: Source/core/dom/shadow/ComposedTreeTraversal.h
diff --git a/Source/core/dom/shadow/ComposedTreeTraversal.h b/Source/core/dom/shadow/ComposedTreeTraversal.h
index ebc792a55317ac33626663a3f0a211dcd0295ff6..4e0c1111b12f292da80d791e2a3a18010b6593ec 100644
--- a/Source/core/dom/shadow/ComposedTreeTraversal.h
+++ b/Source/core/dom/shadow/ComposedTreeTraversal.h
@@ -82,6 +82,9 @@ public:
     // similar to |previous()| but skipping child nodes of the specified node.
     static Node* previousSkippingChildren(const Node&);
 
+    // Like previous, but visits parents before their children.
+    static Node* previousPostOrder(const Node&, const Node* stayWithin = nullptr);
+
     // Composed tree version of |Node::isDescendantOf(other)|. This function
     // returns true if |other| contains |node|, otherwise returns
     // false. If |other| is |node|, this function returns false.