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

chrome/browser/resources/managed_user_passphrase_dialog.css

Index: chrome/browser/resources/managed_user_passphrase_dialog.css
diff --git a/chrome/browser/resources/managed_user_passphrase_dialog.css b/chrome/browser/resources/managed_user_passphrase_dialog.css
new file mode 100644
index 0000000000000000000000000000000000000000..ac538b69315e9ec3ed2258991813e3908f43d95b
--- /dev/null
+++ b/chrome/browser/resources/managed_user_passphrase_dialog.css
@@ -0,0 +1,30 @@
+#family-control-passphrase-page {
+  min-width: 350px;
+}
+
+#instruct {
+  padding: 5px 15px;
+}
+
+#entry {
+  padding: 5px 15px;
+}
+
+#ref {
+  padding: 1px 15px;
+}
+
+#unlock {
+  float: right;
+}
+
+#buttons {
+  padding: 15px 15px;
+}
+
+#incorrect_passphrase_warning {
+  color: red;
+  font-weight: bold;
+  padding: 5px 15px;
+  visibility: hidden;
+}