Handle the case when the last selection is not set yet.

When a user starts to drag an item before they haven't selected any item yet,
FileTable.lastSelection_ can be undefined and this ends up an incorrect drag image.
The existence of lastSelection_ should be checked.

BUG=409514
TEST=manually

Review URL: https://codereview.chromium.org/532453002

Cr-Commit-Position: refs/heads/master@{#292845}

ui/file_manager/file_manager/foreground/js/file_table.js

@@ -395,7 +395,7 @@ FileTable.prototype.shouldStartDragSelection_ = function(event) {
 
   // If the pointed item is already selected, it should not start the drag
   // selection.
-  if (this.lastSelection_.indexOf(itemIndex) !== -1)
+  if (this.lastSelection_ && this.lastSelection_.indexOf(itemIndex) !== -1)
     return false;
 
   // If the horizontal value is not hit to column, it should start the drag