Expose html placeholder attribute as AXPlaceholder: <value> on MAC.

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 218 matching lines @@
     auto_complete,
     container_live_relevant,
     container_live_status,
     description,
     display,
     help,
     html_tag,
     name,
     live_relevant,
     live_status,
+    placeholder,
     role,
     shortcut,
     text_input_type,
     url,
     value
   };
 
   [cpp_enum_prefix_override="ax_attr"] enum AXIntAttribute {
     // Scrollable container attributes.
     scroll_x,
@@ skipping 130 matching lines @@
     word_ends
   };
 
   [cpp_enum_prefix_override="ax"] enum AXTextDirection {
     text_direction_lr,
     text_direction_rl,
     text_direction_tb,
     text_direction_bt
   };
 };