Make NGFragmentBuilder to keep track on text children.

third_party/WebKit/Source/core/layout/ng/ng_box_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 NGBoxFragment_h
 #define NGBoxFragment_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_fragment.h"
 #include "core/layout/ng/ng_physical_box_fragment.h"
 #include "core/layout/ng/ng_writing_mode.h"
 
 namespace blink {
 
 class CORE_EXPORT NGBoxFragment final : public NGFragment {
  public:
   NGBoxFragment(NGWritingMode writing_mode,
-                NGPhysicalBoxFragment* physical_fragment)
+                const NGPhysicalBoxFragment* physical_fragment)
       : NGFragment(writing_mode, physical_fragment) {}
 
   const WTF::Optional<NGLogicalOffset>& BfcOffset() const;
 
   const NGMarginStrut& EndMarginStrut() const;
 };
 
 DEFINE_TYPE_CASTS(NGBoxFragment,
                   NGFragment,
                   fragment,
                   fragment->Type() == NGPhysicalFragment::kFragmentBox,
                   fragment.Type() == NGPhysicalFragment::kFragmentBox);
 
 }  // namespace blink
 
 #endif  // NGBoxFragment_h