[layoutng] Implement state machine for async layout

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_fragment.h"
 #include "core/layout/ng/ng_units.h"
 
 namespace blink {
 
-class CORE_EXPORT NGFragmentBuilder final {
+class CORE_EXPORT NGFragmentBuilder final
+    : public GarbageCollected<NGFragmentBuilder> {
  public:
   NGFragmentBuilder(NGFragmentBase::NGFragmentType);
 
   NGFragmentBuilder& SetWritingMode(NGWritingMode);
   NGFragmentBuilder& SetDirection(NGDirection);
 
   NGFragmentBuilder& SetInlineSize(LayoutUnit);
   NGFragmentBuilder& SetBlockSize(LayoutUnit);
 
   NGFragmentBuilder& SetInlineOverflow(LayoutUnit);
   NGFragmentBuilder& SetBlockOverflow(LayoutUnit);
 
   NGFragmentBuilder& AddChild(const NGFragment*);
 
   // Offsets are not supposed to be set during fragment construction, so we
   // do not provide a setter here.
 
   // Creates the fragment. Can only be called once.
   NGFragment* ToFragment();
 
+  DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(children_); }
+
  private:
   NGFragmentBase::NGFragmentType type_;
   NGWritingMode writing_mode_;
   NGDirection direction_;
 
   NGLogicalSize size_;
   NGLogicalSize overflow_;
 
-  PersistentHeapVector<Member<const NGFragmentBase>> children_;
+  HeapVector<Member<const NGFragmentBase>> children_;
 };
 
 }  // namespace blink
 
 #endif  // NGFragmentBuilder