[LayoutNG] Introduce NGPhysicalFragment and make NGFragment a 'view'

third_party/WebKit/Source/core/layout/ng/ng_box.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 NGBox_h
 #define NGBox_h
 
 #include "core/layout/LayoutBox.h"
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class ComputedStyle;
 class LayoutBox;
 class NGBlockLayoutAlgorithm;
 class NGConstraintSpace;
 class NGFragment;
+class NGPhysicalFragment;
 
 // Represents a node to be laid out.
 class CORE_EXPORT NGBox final : public GarbageCollected<NGBox> {
  public:
   explicit NGBox(LayoutObject*);
 
   // Returns true when done; when this function returns false, it has to be
   // called again. The out parameter will only be set when this function
   // returns true. The same constraint space has to be passed each time.
   // TODO(layout-ng): Should we have a StartLayout function to avoid passing
   // the same space for each Layout iteration?
   bool Layout(const NGConstraintSpace*, NGFragment**);
   const ComputedStyle* Style() const;
 
   NGBox* NextSibling() const;
 
   NGBox* FirstChild() const;
 
+  DEFINE_INLINE_VIRTUAL_TRACE() {
+    visitor->trace(algorithm_);
+    visitor->trace(fragment_);
+  }
+
+ private:
   // This is necessary for interop between old and new trees -- after our parent
   // positions us, it calls this function so we can store the position on the
   // underlying LayoutBox.
-  void PositionUpdated(const NGFragment&);
-
-  DEFINE_INLINE_VIRTUAL_TRACE() { visitor->trace(algorithm_); }
-
- private:
+  void PositionUpdated();
   bool CanUseNewLayout();
 
   LayoutBox* layout_box_;
   Member<NGBlockLayoutAlgorithm> algorithm_;
+  Member<NGPhysicalFragment> fragment_;
 };
 
 }  // namespace blink
 
 #endif  // NGBox_h