Settings WebUI: add aria-expanded to advanced settings toggle button

Bug: 870460
Change-Id: I5a579b584f9fb13a0c1010109b3990ddd4306a0b
Reviewed-on: https://chromium-review.googlesource.com/c/1308601Reviewed-by: Hector Carmona <hcarmona@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604732}

chrome/browser/resources/settings/basic_page/basic_page.html

@@ -220,9 +220,8 @@
           inSearchMode, hasExpandedSection_)]]">
         <div id="toggleSpacer"></div>
         <h2 id="toggleContainer">
-          <paper-button id="advancedToggle"
-              aria-active-attribute="aria-expanded"
-              on-click="advancedToggleClicked_">
+          <paper-button id="advancedToggle" on-click="advancedToggleClicked_"
+              aria-expanded$="[[boolToString_(advancedToggleExpanded)]]">
             <span>$i18n{advancedPageTitle}</span>
             <iron-icon icon="[[getArrowIcon_(advancedToggleExpanded)]]">
             </iron-icon>

chrome/browser/resources/settings/basic_page/basic_page.js

@@ -387,4 +387,13 @@ Polymer({
   getArrowIcon_: function(opened) {
     return opened ? 'cr:arrow-drop-up' : 'cr:arrow-drop-down';
   },
+
+  /**
+   * @param {boolean} bool
+   * @return {string}
+   * @private
+   */
+  boolToString_: function(bool) {
+    return bool.toString();
+  },
 });