Make NGFragmentBuilder to keep track on text children.

third_party/WebKit/Source/core/layout/ng/ng_fragment.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 NGFragment_h
 #define NGFragment_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_physical_fragment.h"
 #include "core/layout/ng/ng_writing_mode.h"
@@ skipping 18 matching lines @@
   LayoutUnit InlineOverflow() const;
   LayoutUnit BlockOverflow() const;
 
   // Returns the offset relative to the parent fragment's content-box.
   LayoutUnit InlineOffset() const;
   LayoutUnit BlockOffset() const;
 
   NGPhysicalFragment::NGFragmentType Type() const;
 
  protected:
-  NGFragment(NGWritingMode writing_mode, NGPhysicalFragment* physical_fragment)
+  NGFragment(NGWritingMode writing_mode,
+             const NGPhysicalFragment* physical_fragment)
       : physical_fragment_(physical_fragment), writing_mode_(writing_mode) {}
 
-  NGPhysicalFragment* physical_fragment_;
+  const NGPhysicalFragment* physical_fragment_;
 
   unsigned writing_mode_ : 3;
 };
 
 }  // namespace blink
 
 #endif  // NGFragment_h