[Galley] Fix an exception in clicking a thumbnail

This patch fixes the exception in clicking a thumbnail. It is a regression caused by r249027 and a possible cause of crbug.com/340624.

The 'length' property of the selection model is NOT the number of the selected item. To get it, we need to get the length of 'selectedIndex' property.

BUG=340624
TEST=manually

Review URL: https://codereview.chromium.org/135973012

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251290 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/resources/file_manager/foreground/js/photo/gallery.js

@@ -637,13 +637,12 @@ Gallery.prototype.onKeyDown_ = function(event) {
  * @private
  */
 Gallery.prototype.updateSelectionAndState_ = function() {
-  var numSelectedItems = this.selectionModel_.length;
+  var numSelectedItems = this.selectionModel_.selectedIndexes.length;
   var displayName = '';
   var selectedEntryURL = null;
 
   // If it's selecting something, update the variable values.
   if (numSelectedItems) {
-    var selectedIndex = this.selectionModel_.selectedIndex;
     var selectedItem =
         this.dataModel_.item(this.selectionModel_.selectedIndex);
     this.selectedEntry_ = selectedItem.getEntry();