[LayoutNG] Switch NGBreakToken to being RefCounted.

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

 // Copyright 2017 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 NGBlockChildIterator_h
 #define NGBlockChildIterator_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
+#include "wtf/RefPtr.h"
 
 namespace blink {
 
 class NGLayoutInputNode;
 class NGBreakToken;
 class NGBlockBreakToken;
 
 // A utility class for block-flow layout which given the first child and a
 // break token will iterate through unfinished children.
 //
 // This class does not handle modifications to its arguments after it has been
 // constructed.
 class CORE_EXPORT NGBlockChildIterator {
+  STACK_ALLOCATED();
+
  public:
   NGBlockChildIterator(NGLayoutInputNode* first_child,
                        NGBlockBreakToken* break_token);
 
   // Returns the next input node which should be laid out, along with its
   // respective break token.
   struct Entry;
   Entry NextChild();
 
  private:
   Persistent<NGLayoutInputNode> child_;
-  Persistent<NGBlockBreakToken> break_token_;
+  NGBlockBreakToken* break_token_;  // Not-owned as STACK_ALLOCATED.

[ikilpatrick, 2017/02/28 17:13:13]

it'd be nice if we had a class like:
NotARefPtrBecuaseStackAllocated<>

 
   // An index into break_token_'s ChildBreakTokens() vector. Used for keeping
   // track of the next child break token to inspect.
   size_t child_token_idx_;
 };
 
 struct NGBlockChildIterator::Entry {
+  STACK_ALLOCATED();
+
   Entry(NGLayoutInputNode* node, NGBreakToken* token)
       : node(node), token(token) {}
 
   Persistent<NGLayoutInputNode> node;
-  Persistent<NGBreakToken> token;
+  NGBreakToken* token;  // Not-owned as STACK_ALLOCATED.
 
   bool operator==(const NGBlockChildIterator::Entry& other) const {
     return node == other.node && token == other.token;
   }
 };
 
 }  // namespace blink
 
 #endif  // NGBlockChildIterator_h