Support aria-grabbed attribute.

public/web/WebAXEnums.h

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 222 matching lines @@
 
 // Expanded State.
 // These values must match blink::AccessibilityExpanded values.
 // Enforced in AssertMatchingEnums.cpp.
 enum WebAXExpanded {
     WebAXExpandedUndefined = 0,
     WebAXExpandedCollapsed,
     WebAXExpandedExpanded
 };
 
+// Attribute definition values.
+// These values must match blink::AccessibilityOptionalBool values.
+// Enforced in AssertMatchingEnums.cpp.
+enum WebAXOptionalBool {
+    WebAXOptionalBoolUndefined = 0,
+    WebAXOptionalBoolTrue,
+    WebAXOptionalBoolFalse
+};
+
 } // namespace blink
 
 #endif