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

Source/core/layout/svg/LayoutSVGGradientStop.h

 /*
  * Copyright (C) 2007 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2009 Google, Inc.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 24 matching lines @@
     virtual ~LayoutSVGGradientStop();
 
     virtual const char* name() const override { return "LayoutSVGGradientStop"; }
     virtual bool isOfType(LayoutObjectType type) const override { return type == LayoutObjectSVG || type == LayoutObjectSVGGradientStop || LayoutObject::isOfType(type); }
 
     virtual void layout() override;
 
     // This overrides are needed to prevent ASSERTs on <svg><stop /></svg>
     // LayoutObject's default implementations ASSERT_NOT_REACHED()
     // https://bugs.webkit.org/show_bug.cgi?id=20400
-    virtual LayoutRect clippedOverflowRectForPaintInvalidation(const LayoutBoxModelObject*, const PaintInvalidationState* = 0) const override { return LayoutRect(); }
+    virtual LayoutRect clippedOverflowRectForPaintInvalidation(const LayoutBoxModelObject*, const PaintInvalidationState* = nullptr) const override { return LayoutRect(); }
     virtual FloatRect objectBoundingBox() const override { return FloatRect(); }
     virtual FloatRect strokeBoundingBox() const override { return FloatRect(); }
     virtual FloatRect paintInvalidationRectInLocalCoordinates() const override { return FloatRect(); }
 
 protected:
     virtual void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
 
 private:
     SVGGradientElement* gradientElement() const;
 };
 
 DEFINE_LAYOUT_OBJECT_TYPE_CASTS(LayoutSVGGradientStop, isSVGGradientStop());
 
 }
 
 #endif // LayoutSVGGradientStop_h