Fixed issue with miscalculated aria-posinset attribute's value.

This bug was caused by returning a string as an index from list_selection_model.js in an Event argument of a change event. In the list.js, such index stored as string was incremented, which caused concatenation instead of incrementation. This patch converts these values to number to avoid such problems.

TEST=Enable ChromeVox, enter Files.app (CTRL-M), navigate among files. Correct item number should be spoken, eg. 2/x, 3/x.
BUG=164061


Review URL: https://chromiumcodereview.appspot.com/11421214

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@171056 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/shared/js/cr/ui/list_selection_model.js

@@ -227,7 +227,7 @@ cr.define('cr.ui', function() {
           var e = new Event('change');
           e.changes = indexes.map(function(index) {
             return {
-              index: index,
+              index: Number(index),
               selected: this.changedIndexes_[index]
             };
           }, this);