[LayoutNG] Switch NGConstraintSpace to being RefCounted.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NGLayoutOpportunityIterator_h
 #define NGLayoutOpportunityIterator_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_constraint_space.h"
 #include "core/layout/ng/ng_layout_opportunity_tree_node.h"
@@ skipping 34 matching lines @@
   // Mutable Getters.
   NGLayoutOpportunityTreeNode* MutableOpportunityTreeRoot() {
     return opportunity_tree_root_.get();
   }
 
   // Read-only Getters.
   const NGLayoutOpportunityTreeNode* OpportunityTreeRoot() const {
     return opportunity_tree_root_.get();
   }
 
-  Persistent<NGConstraintSpace> constraint_space_;
+  NGConstraintSpace* constraint_space_;
 
   NGLayoutOpportunities opportunities_;
   NGLayoutOpportunities::const_iterator opportunity_iter_;
   Persistent<NGLayoutOpportunityTreeNode> opportunity_tree_root_;
 };
 
 }  // namespace blink
 
 #endif  // NGLayoutOpportunityIterator_h