Add ShowLayoutOpportunityTree to NGLayoutOpportunityIterator

third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h

Index: third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h
index 6e2c913b6db0183d521b75d459f548bb528ee63c..ea12337bc0895c4aef6281bbd81357d91fe1b5b2 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h
+++ b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.h
@@ -48,8 +48,20 @@ struct CORE_EXPORT NGLayoutOpportunityTreeNode
   // The node is a leaf if it doesn't have an exclusion that splits it apart.
   bool IsLeafNode() const { return !exclusion; }
 
+  String ToString() const;
+
   DECLARE_TRACE();
 };
 
+inline std::ostream& operator<<(std::ostream& stream,
+                                const NGLayoutOpportunityTreeNode& value) {
+  return stream << value.ToString();
+}
+
+inline std::ostream& operator<<(std::ostream& out,
+                                const NGLayoutOpportunityTreeNode* value) {
+  return out << (value ? value->ToString() : "(null)");
+}
+
 }  // namespace blink
 #endif  // NGLayoutOpportunityTreeNode_h