Support aria-dropeffect attribute.

Source/modules/accessibility/AXNodeObject.h

 /*
  * Copyright (C) 2012, Google 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 133 matching lines @@
     virtual String ariaInvalidValue() const override final;
     virtual String valueDescription() const override;
     virtual float valueForRange() const override;
     virtual float maxValueForRange() const override;
     virtual float minValueForRange() const override;
     virtual String stringValue() const override;
     virtual const AtomicString& textInputType() const override;
 
     // ARIA attributes.
     virtual String ariaDescribedByAttribute() const override final;
+    virtual const AtomicString& ariaDropEffect() const override final;
     virtual String ariaLabeledByAttribute() const override final;
     virtual AccessibilityRole ariaRoleAttribute() const override final;
     virtual AccessibilityOptionalBool isAriaGrabbed() const override final;
 
     // Accessibility Text.
     virtual String textUnderElement() const override;
     virtual String accessibilityDescription() const override;
     virtual String title() const override;
     virtual String helpText() const override;
     virtual String computedName() const override;
@@ skipping 39 matching lines @@
     void changeValueByPercent(float percentChange);
     float stepValueForRange() const;
     AXObject* findChildWithTagName(const HTMLQualifiedName&) const;
 };
 
 DEFINE_AX_OBJECT_TYPE_CASTS(AXNodeObject, isAXNodeObject());
 
 } // namespace blink
 
 #endif // AXNodeObject_h