Ensure the correct wallpaper is loaded when Add User screen is closed

If |focusedPod_| is already set, and it is the same with |podToFocus|,
early return is hit in focusPod(), however we should force a focus
update in order to ensure the correct wallpaper is set.

Bug: 769492
Change-Id: I820a523c863d693663411e436be2baa8b62fa178
Reviewed-on: https://chromium-review.googlesource.com/691046Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505409}

ui/login/account_picker/md_user_pod_row.js

@@ -4783,7 +4783,8 @@ cr.define('login', function() {
      */
     maybePreselectPod: function() {
       var pod = this.preselectedPod;
-      this.focusPod(pod);
+      // Force a focus update to ensure the correct wallpaper is loaded.
+      this.focusPod(pod, true /* force */);
 
       // Hide user-type-bubble in case all user pods are disabled and we focus
       // first pod.