[LayoutNG] Switch NGBreakToken to being RefCounted.

third_party/WebKit/Source/core/layout/ng/ng_fragment_builder.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 NGFragmentBuilder_h
 #define NGFragmentBuilder_h
 
 #include "core/layout/ng/ng_break_token.h"
 #include "core/layout/ng/ng_constraint_space.h"
 #include "core/layout/ng/ng_floating_object.h"
@@ skipping 135 matching lines @@
 
   NGLogicalSize size_;
   NGLogicalSize overflow_;
 
   Vector<RefPtr<NGPhysicalFragment>> children_;
   Vector<NGLogicalOffset> offsets_;
 
   bool did_break_;
   LayoutUnit used_block_size_;
 
-  Persistent<HeapVector<Member<NGBreakToken>>> child_break_tokens_;
+  Vector<RefPtr<NGBreakToken>> child_break_tokens_;
 
   WeakBoxList out_of_flow_descendant_candidates_;
   Vector<OutOfFlowPlacement> out_of_flow_candidate_placements_;
 
   WeakBoxList out_of_flow_descendants_;
   Vector<NGStaticPosition> out_of_flow_positions_;
 
   // Floats that need to be positioned by the next in-flow fragment that can
   // determine its block position in space.
   Vector<Persistent<NGFloatingObject>> unpositioned_floats_;
 
   Vector<NGLogicalOffset> floating_object_offsets_;
   Vector<Persistent<NGFloatingObject>> positioned_floats_;
 
   WTF::Optional<NGLogicalOffset> bfc_offset_;
   NGMarginStrut end_margin_strut_;
 };
 
 }  // namespace blink
 
 #endif  // NGFragmentBuilder