Implement important sites dialog for desktop.

chrome/browser/resources/settings/controls/important_site_checkbox.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.
+
+/**
+ * @fileoverview
+ * `important-site-checkbox` is a checkbox that displays an important site.
+ * An important site represents a domain with data that the user might want
+ * to protect from being deleted. ImportantSites are determined based on
+ * various engagement factors, such as whether a site is bookmarked or receives
+ * notifications.
+ */
+Polymer({
+  is: 'important-site-checkbox',
+
+  properties: {
+    /** @type {ImportantSite} */
+    site: Object,
+
+    disabled: {type: Boolean, value: false},
+  }
+});