Add a screen to migrate filesystem encryption from eCryptfs to ext4 crypto.

chrome/browser/resources/chromeos/login/screen_encryption_migration.js

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+login.createScreen('EncryptionMigrationScreen', 'encryption-migration',
+    function() {
+  return {
+    EXTERNAL_API: [
+    ],
+
+    /**
+     * Ignore any accelerators the user presses on this screen.
+     */
+    ignoreAccelerators: true,
+
+    /** @override */
+    decorate: function() {
+    },
+
+    /**
+     * Event handler that is invoked just before the screen in shown.
+     */
+    onBeforeShow: function() {
+      $('progress-dots').hidden = true;
+      var headerBar = $('login-header-bar');
+      headerBar.allowCancel = false;
+      headerBar.showGuestButton = false;
+      headerBar.showCreateSupervisedButton = false;
+      headerBar.signinUIState = SIGNIN_UI_STATE.HIDDEN;
+    },
+  };
+});