MD Settings: Add mechanism to restore focus after subpage exiting.

chrome/test/data/webui/settings/settings_animated_pages_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('settings-animated-pages', function() {
+  test('focuses subpage trigger when exiting subpage', function(done) {
+    document.body.innerHTML = `
+      <settings-animated-pages section="test-section">
+        <neon-animatable route-path="default">
+          <button id="subpage-trigger"></button>
+        </neon-animatable>
+        <neon-animatable route-path="${settings.Route.SEARCH_ENGINES.path}">
+          <button id="subpage-trigger"></button>
+        </neon-animatable>
+      </settings-animated-pages>`;
+
+    var animatedPages = document.body.querySelector('settings-animated-pages');
+    animatedPages.focusConfig = new Map();
+    animatedPages.focusConfig.set(
+        settings.Route.SEARCH_ENGINES.path, '#subpage-trigger');
+
+    animatedPages.$.animatedPages.selected = settings.Route.SEARCH_ENGINES.path;
+
+    var trigger = document.body.querySelector('#subpage-trigger');
+    assertTrue(!!trigger);
+    trigger.addEventListener('focus', function() { done(); });
+
+    // Trigger subpage exit.
+    animatedPages.currentRouteChanged(
+        settings.Route.BASIC, settings.Route.SEARCH_ENGINES);
+  });
+});