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

Source/core/layout/svg/LayoutSVGGradientStop.h

Index: Source/core/layout/svg/LayoutSVGGradientStop.h
diff --git a/Source/core/layout/svg/LayoutSVGGradientStop.h b/Source/core/layout/svg/LayoutSVGGradientStop.h
index a448a8e55676f60d6906aa90a453dfbbb2cbc80a..69316c275f8df7936b0a28a16179669c7cc01f0d 100644
--- a/Source/core/layout/svg/LayoutSVGGradientStop.h
+++ b/Source/core/layout/svg/LayoutSVGGradientStop.h
@@ -42,7 +42,7 @@ public:
     // 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(); }