Chrome OS OOBE display manager: DISPLAY_TYPE enum should be global.

This Cl fixes regression added in https://crrev.com/c/1554042 moving
DISPLAY_TYPE enum back to global space.

Bug: 952223
Change-Id: If7dde9e34b18d282e2c258a69d107c7edb7951a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614108Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Alexander Alekseev <alemate@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660609}

ui/login/display_manager_types.js

@@ -74,7 +74,7 @@ DisplayManagerScreenAttributes.toggleKioskAllowed;
  * Possible types of UI.
  * @enum {string}
  */
-const DISPLAY_TYPE = {
+var DISPLAY_TYPE = {
   UNKNOWN: 'unknown',
   OOBE: 'oobe',
   LOGIN: 'login',