Add ShowLayoutOpportunityTree to NGLayoutOpportunityIterator

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

Index: third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.cc
diff --git a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.cc b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.cc
index aa211bf5adf7bf24571ceeb3d842638201717e4c..49b04cd0ea746d617c0d4040726f8e4458d927f8 100644
--- a/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.cc
+++ b/third_party/WebKit/Source/core/layout/ng/ng_layout_opportunity_tree_node.cc
@@ -20,6 +20,13 @@ NGLayoutOpportunityTreeNode::NGLayoutOpportunityTreeNode(
       exclusion_edge(exclusion_edge),
       exclusion(nullptr) {}
 
+String NGLayoutOpportunityTreeNode::ToString() const {
+  return String::format(
+      "Opportunity: '%s' Exclusion: '%s'",
+      opportunity.ToString().ascii().data(),
+      exclusion ? exclusion->ToString().ascii().data() : "null");
+}
+
 DEFINE_TRACE(NGLayoutOpportunityTreeNode) {
   visitor->trace(left);
   visitor->trace(bottom);