Fix <dialog> centering

Source/core/html/HTMLDialogElement.h

Index: Source/core/html/HTMLDialogElement.h
diff --git a/Source/core/html/HTMLDialogElement.h b/Source/core/html/HTMLDialogElement.h
index 9c9782d91ad3c105043bf8443fc4443c421a9c98..091dac64cf6ebf2035551d302b7879fcf9795901 100644
--- a/Source/core/html/HTMLDialogElement.h
+++ b/Source/core/html/HTMLDialogElement.h
@@ -45,8 +45,12 @@ public:
 private:
     HTMLDialogElement(const QualifiedName&, Document*);
 
-    virtual RenderObject* createRenderer(RenderArena*, RenderStyle*) OVERRIDE;
+    virtual PassRefPtr<RenderStyle> customStyleForRenderer() OVERRIDE;
     virtual bool isPresentationAttribute(const QualifiedName&) const OVERRIDE;
+    void positionAndReattach();
+
+    bool m_topIsValid;
+    LayoutUnit m_top;
 };
 
 } // namespace WebCore