C++11: Replace 0 with nullptr where applicable in layout code.

Source/core/layout/LayoutScrollbar.h

 /*
  * Copyright (C) 2008, 2009 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 23 matching lines @@
 namespace blink {
 
 class LocalFrame;
 class Node;
 class LayoutBox;
 class LayoutScrollbarPart;
 class ComputedStyle;
 
 class LayoutScrollbar final : public Scrollbar {
 public:
-    static PassRefPtrWillBeRawPtr<Scrollbar> createCustomScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame* owningFrame = 0);
+    static PassRefPtrWillBeRawPtr<Scrollbar> createCustomScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame* owningFrame = nullptr);
     virtual ~LayoutScrollbar();
 
     LayoutBox* owningLayoutObject() const;
 
     IntRect buttonRect(ScrollbarPart);
     IntRect trackRect(int startLength, int endLength);
     IntRect trackPieceRectWithMargins(ScrollbarPart, const IntRect&);
 
     int minimumThumbLength();
 
@@ skipping 35 matching lines @@
     RawPtrWillBeMember<LocalFrame> m_owningFrame;
 
     HashMap<unsigned, LayoutScrollbarPart*> m_parts;
 };
 
 DEFINE_TYPE_CASTS(LayoutScrollbar, ScrollbarThemeClient, scrollbar, scrollbar->isCustomScrollbar(), scrollbar.isCustomScrollbar());
 
 } // namespace blink
 
 #endif // LayoutScrollbar_h