Implement proposed shadow tree cascade order.

Source/core/dom/DocumentOrderedList.h

Index: Source/core/dom/DocumentOrderedList.h
diff --git a/Source/core/dom/DocumentOrderedList.h b/Source/core/dom/DocumentOrderedList.h
index 7cc6d0f14487e8b128504a9fd54672d467ec372e..0acb91bd2d2d75c85d0e5e304ce3bb970f8753b5 100644
--- a/Source/core/dom/DocumentOrderedList.h
+++ b/Source/core/dom/DocumentOrderedList.h
@@ -50,10 +50,14 @@ public:
     size_t size() const { return m_nodes.size(); }
 
     using iterator = WillBeHeapListHashSet<RawPtrWillBeMember<Node>, 32>::iterator;
+    using const_reverse_iterator = WillBeHeapListHashSet<RawPtrWillBeMember<Node>, 32>::const_reverse_iterator;
 
     iterator begin() { return m_nodes.begin(); }
     iterator end() { return m_nodes.end(); }
 
+    const_reverse_iterator rbegin() const { return m_nodes.rbegin(); }
+    const_reverse_iterator rend() const { return m_nodes.rend(); }
+
     DECLARE_TRACE();
 
 private: