Use correct position when setting pos_in_set for tabs.

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.
 
 // TODO(nektar): Migrate entire file to Mojoq.
 // 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 four enums, also add to
@@ skipping 392 matching lines @@
     // Relationships between this element and other elements.
     activedescendant_id,
     errormessage_id,
     member_of_id,
     next_on_line_id,
     previous_on_line_id,
 
     // 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.
     pos_in_set,
 
     // In the case of AX_ROLE_COLOR_WELL, specifies the selected color.
     color_value,
 
     // Indicates the element that represents the current item within a container
     // or set of related elements.
     aria_current_state,
 
     // Text attributes.
@@ skipping 228 matching lines @@
     // First node is before the second one.
     before,
 
     // Nodes are the same.
     equal,
 
     // First node is after the second one.
     after
   };
 };