Add a lock to the managed user settings page and require authentication for unlocking.

chrome/browser/resources/options/managed_user_set_passphrase.css

Index: chrome/browser/resources/options/managed_user_set_passphrase.css
diff --git a/chrome/browser/resources/options/alert_overlay.css b/chrome/browser/resources/options/managed_user_set_passphrase.css
similarity index 75%
copy from chrome/browser/resources/options/alert_overlay.css
copy to chrome/browser/resources/options/managed_user_set_passphrase.css
index b2d9960a9a1554c657e841e8b1f3b1131114c581..511c42c1841b32ed69251f127521aaddb04ae8ae 100644
--- a/chrome/browser/resources/options/alert_overlay.css
+++ b/chrome/browser/resources/options/managed_user_set_passphrase.css
@@ -2,6 +2,6 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */
 
-#alertOverlayMessage {
-  width: 400px;
+#managed-user-set-passphrase-page {
+  min-width: 400px;
 }