Handles aria-dropeffect after getting its value from blink.

ui/accessibility/ax_enums.idl

 // 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.
 
 // These should be kept in sync with third_party/WebKit/public/web/WebAXEnums.h
 // until the Chromium and Blink trees are merged.
 [camel_case_enum_to_string=true] namespace ui {
 
   // For new entries to the following three enums, also add to
   // chrome/common/extensions/api/automation.idl.
@@ skipping 215 matching lines @@
     doc_doctype,
 
     // Attributes that could apply to any node.
     access_key,
     action,
     auto_complete,
     container_live_relevant,
     container_live_status,
     description,
     display,
+    dropeffect,
     help,
     html_tag,
     // Only used when invalid_state == invalid_state_other.
     aria_invalid_value,
     name,
     live_relevant,
     live_status,
     placeholder,
     role,
     shortcut,
@@ skipping 155 matching lines @@
 
   [cpp_enum_prefix_override="ax"] enum AXInvalidState {
     invalid_state_false,
     invalid_state_true,
     invalid_state_spelling,
     invalid_state_grammar,
     invalid_state_other
   };
 
 };