Implement DOMQuad of geometry interfaces.

Source/core/dom/DOMPointReadOnly.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef DOMPointReadOnly_h
 #define DOMPointReadOnly_h
 
 #include "platform/heap/Handle.h"
 
 namespace WebCore {
 
 class DOMPointReadOnly : public GarbageCollected<DOMPointReadOnly> {
 public:
     static DOMPointReadOnly* create(double x, double y, double z, double w);
 
     double x() const { return m_x; }
     double y() const { return m_y; }
     double z() const { return m_z; }
     double w() const { return m_w; }
 
-    void trace(Visitor*) { }
+    virtual void trace(Visitor*) { }
 
 protected:
     DOMPointReadOnly(double x, double y, double z, double w);
 
     double m_x;
     double m_y;
     double m_z;
     double m_w;
 };
 
 } // namespace WebCore
 
 #endif