Make NGFragmentBuilder to keep track on text children.

third_party/WebKit/Source/core/layout/ng/ng_text_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 NGTextFragment_h
 #define NGTextFragment_h
 
 #include "core/CoreExport.h"
 #include "core/layout/ng/ng_fragment.h"
 #include "core/layout/ng/ng_physical_text_fragment.h"
 
 namespace blink {
 
 class CORE_EXPORT NGTextFragment final : public NGFragment {
  public:
   NGTextFragment(NGWritingMode writing_mode,
-                 NGPhysicalTextFragment* physical_text_fragment)
+                 const NGPhysicalTextFragment* physical_text_fragment)
       : NGFragment(writing_mode, physical_text_fragment) {}
 };
 
 }  // namespace blink
 
 #endif  // NGTextFragment_h