cros: Remove white background before showing web-ui login

1) Verified with --show-webui-login on multiple cases to make sure M69
   is working.

2) Implementation of transitioning between OOBE and views login is WIP.
   It is a must-have before deprecating web-ui login code.

Bug: 875128
Change-Id: I1e5cf08d9c978834830ca0d952f61b26b558a7b7
Reviewed-on: https://chromium-review.googlesource.com/1180475Reviewed-by: Jacob Dufault <jdufault@chromium.org>
Reviewed-by: Alexander Alekseev <alemate@chromium.org>
Commit-Queue: Wenzhao (Colin) Zang <wzang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584240}

ui/login/account_picker/md_user_pod_row.js

@@ -4917,6 +4917,9 @@ cr.define('login', function() {
       }
 
       this.handleAfterPodPlacement_();
+      // This is a hack for https://crbug.com/875128.
+      if (Oobe.getInstance().displayType == DISPLAY_TYPE.OOBE)
+        document.documentElement.removeAttribute('full-screen-dialog');
     },
 
     /**