MD Settings: fix auto-focus on sync page.

chrome/test/data/webui/settings/settings_sync_page_test.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.
+
+suite('sync-page-test', function() {
+  /** @type {SyncPageElement} */ var testElement;
+
+  setup(function() {
+    PolymerTest.clearBody();
+
+    testElement = document.createElement('settings-sync-page');
+    document.body.appendChild(testElement);
+  });
+
+  test('autofocus correctly after container is shown', function() {
+    cr.webUIListenerCallback('sync-prefs-changed', {passphraseRequired: true});
+    Polymer.dom.flush();
+
+    var input = testElement.$$('#existingPassphraseInput');
+
+    var focused = false;
+    input.addEventListener('focus', function() {
+      focused = true;
+    });
+
+    testElement.fire('show-container');

[dpapad, 2017/06/14 01:02:00]

Nit: Maybe add a short comment 

// Simulate event normally fired by ...

+    assertTrue(focused);
+  });
+});