Add AX role ,'AXDescriptionList', for dl element(re-land).

Source/core/accessibility/AXList.h

 /*
  * Copyright (C) 2008 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 25 matching lines @@
 class AXList FINAL : public AXRenderObject {
 
 private:
     explicit AXList(RenderObject*);
 public:
     static PassRefPtr<AXList> create(RenderObject*);
     virtual ~AXList();
 
     virtual bool isList() const OVERRIDE { return true; }
 
-    virtual AccessibilityRole roleValue() const OVERRIDE { return ListRole; }
+    virtual AccessibilityRole roleValue() const OVERRIDE FINAL;
 private:
+    bool isDescriptionList() const;
     virtual bool computeAccessibilityIsIgnored() const OVERRIDE;
 };
 
 } // namespace blink
 
 #endif // AXList_h