Support aria-dropeffect attribute.

Source/modules/accessibility/AXNodeObject.cpp

 /*
 * 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 1204 matching lines @@
 }
 
 String AXNodeObject::ariaDescribedByAttribute() const
 {
     WillBeHeapVector<RawPtrWillBeMember<Element>> elements;
     elementsFromAttribute(elements, aria_describedbyAttr);
 
     return accessibilityDescriptionForElements(elements);
 }
 
+const AtomicString& AXNodeObject::ariaDropEffect() const
+{
+    return getAttribute(aria_dropeffectAttr);
+}
+
 String AXNodeObject::ariaLabeledByAttribute() const
 {
     WillBeHeapVector<RawPtrWillBeMember<Element>> elements;
     ariaLabeledByElements(elements);
 
     return accessibilityDescriptionForElements(elements);
 }
 
 AccessibilityRole AXNodeObject::ariaRoleAttribute() const
 {
@@ skipping 724 matching lines @@
     float range = maxValueForRange() - minValueForRange();
     float value = valueForRange();
 
     value += range * (percentChange / 100);
     setValue(String::number(value));
 
     axObjectCache()->postNotification(node(), AXObjectCacheImpl::AXValueChanged, true);
 }
 
 } // namespace blink