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

Source/core/layout/LayoutScrollbarPart.h

Index: Source/core/layout/LayoutScrollbarPart.h
diff --git a/Source/core/layout/LayoutScrollbarPart.h b/Source/core/layout/LayoutScrollbarPart.h
index 488d23bee0f4c234738133370a2d168b35bee684..1fd8165e55dfcd098e44ebe0f1f141c9f898eda5 100644
--- a/Source/core/layout/LayoutScrollbarPart.h
+++ b/Source/core/layout/LayoutScrollbarPart.h
@@ -35,7 +35,7 @@ class LayoutScrollbar;
 
 class LayoutScrollbarPart final : public LayoutBlock {
 public:
-    static LayoutScrollbarPart* createAnonymous(Document*, LayoutScrollbar* = 0, ScrollbarPart = NoPart);
+    static LayoutScrollbarPart* createAnonymous(Document*, LayoutScrollbar* = nullptr, ScrollbarPart = NoPart);
 
     virtual ~LayoutScrollbarPart();
 
@@ -78,7 +78,7 @@ public:
 protected:
     virtual void styleWillChange(StyleDifference, const ComputedStyle& newStyle) override;
     virtual void styleDidChange(StyleDifference, const ComputedStyle* oldStyle) override;
-    virtual void imageChanged(WrappedImagePtr, const IntRect* = 0) override;
+    virtual void imageChanged(WrappedImagePtr, const IntRect* = nullptr) override;
 
 private:
     LayoutScrollbarPart(LayoutScrollbar*, ScrollbarPart);