Files app: Fix FileGrid.Item as JS module

When running as JS modules which is strict by default the cr.ui.List
fails when trying to set the label here [1].  Fix by adding a no-op
setter in the FileGrid.Item.

[1] -
https://source.chromium.org/chromium/chromium/src/+/master:ui/webui/resources/js/cr/ui/list.js;l=898;drc=d95bb037d381ef68fa70418e2a8dedce62c1127b

Bug: 1133186
Change-Id: Idc0a97d8739c029dc1709d8c90bd20aaf786fd57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2636742
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: Jeremie Boulic <jboulic@chromium.org>
Reviewed-by: Jeremie Boulic <jboulic@chromium.org>
Auto-Submit: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844683}

ui/file_manager/file_manager/foreground/js/ui/file_grid.js

@@ -1039,6 +1039,10 @@ FileGrid.Item = class extends cr.ui.ListItem {
     return this.querySelector('filename-label').textContent;
   }
 
+  set label(newLabel) {
+    // no-op setter. cr.ui.List calls this setter but Files app doesn't need it.
+  }
+
   /**
    * @override
    */