ARIA state/property aria-autocomplete should expose IA2_STATE_SUPPORTS_AUTOCOMPLETION.

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 207 matching lines @@
   [cpp_enum_prefix_override="ax_attr"] enum AXStringAttribute {
     // Document attributes.
     doc_url,
     doc_title,
     doc_mimetype,
     doc_doctype,
 
     // Attributes that could apply to any node.
     access_key,
     action,
+    auto_complete,
     container_live_relevant,
     container_live_status,
     description,
     display,
     help,
     html_tag,
     name,
     live_relevant,
     live_status,
     role,
@@ skipping 139 matching lines @@
     word_ends
   };
 
   [cpp_enum_prefix_override="ax"] enum AXTextDirection {
     text_direction_lr,
     text_direction_rl,
     text_direction_tb,
     text_direction_bt
   };
 };