Use correct position when setting pos_in_set for tabs.

ui/accessibility/ax_enums.idl

Index: ui/accessibility/ax_enums.idl
diff --git a/ui/accessibility/ax_enums.idl b/ui/accessibility/ax_enums.idl
index 2016c2419ef1d90afc6b4fe1862df727a23e50f0..88181b4c2f41faba90b9af97e4271b6eeb08f963 100644
--- a/ui/accessibility/ax_enums.idl
+++ b/ui/accessibility/ax_enums.idl
@@ -410,8 +410,11 @@
     // Identifies a child tree which this node hosts.
     child_tree_id,
 
-    // Position or Number of items in current set of listitems or treeitems
+    // Number of items in a set typically of treeitems orlistitems.
     set_size,
+
+    // The 1-based position of the item within the set above. Do not set
+    // |index_in_set|.
     pos_in_set,
 
     // In the case of AX_ROLE_COLOR_WELL, specifies the selected color.
@@ -438,6 +441,12 @@
     text_style
   };
 
+  [cpp_enum_prefix_override="ax_attr"] enum AXValidatedIntAttribute {
+    // The 0-based position of the item within the set of |set_size| items. Do
+    // not set |pos_in_set|.
+    index_in_set,
+  };
+
   [cpp_enum_prefix_override="ax_attr"] enum AXFloatAttribute {
     // Range attributes.
     value_for_range,