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 beeing 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},
+  },
+
+  /**
+   * Returns the icon to use for a given site.
+   * @param {string} url The url of the site to fetch the icon for.
+   * @return {string} The background-image style with the favicon.
+   * @private
+   */
+  siteIcon_: function(url) {
+    return 'background-image: ' + cr.icon.getFavicon(url);
+  },
+});