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

content/browser/accessibility/accessibility_tree_formatter_mac.mm

 // Copyright (c) 2012 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.
 
 #include "content/browser/accessibility/accessibility_tree_formatter.h"
 
 #import <Cocoa/Cocoa.h>
 
 #include "base/basictypes.h"
 #include "base/files/file_path.h"
@@ skipping 151 matching lines @@
       @"AXARIALive",
       @"AXARIARelevant",
       NSAccessibilityColumnIndexRangeAttribute,
       NSAccessibilityEnabledAttribute,
       NSAccessibilityFocusedAttribute,
       NSAccessibilityIndexAttribute,
       @"AXLoaded",
       @"AXLoadingProcess",
       NSAccessibilityNumberOfCharactersAttribute,
       NSAccessibilityOrientationAttribute,
+      @"AXPlaceholder",
       @"AXRequired",
       NSAccessibilityRowIndexRangeAttribute,
       NSAccessibilitySelectedChildrenAttribute,
       NSAccessibilityTitleUIElementAttribute,
       NSAccessibilityURLAttribute,
       NSAccessibilityVisibleCharacterRangeAttribute,
       NSAccessibilityVisibleChildrenAttribute,
       @"AXVisited",
       @"AXLinkedUIElements",
       NSAccessibilityExpandedAttribute,
@@ skipping 127 matching lines @@
 const string AccessibilityTreeFormatter::GetAllowString() {
   return "@MAC-ALLOW:";
 }
 
 // static
 const string AccessibilityTreeFormatter::GetDenyString() {
   return "@MAC-DENY:";
 }
 
 }  // namespace content