Allow the 'Advanced' button to act as an expandable toggle for a11y.

chrome/browser/resources/settings/settings_main/settings_main.html

 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/promise_resolver.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-icon/iron-icon.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="/about_page/about_page.html">
 <link rel="import" href="/advanced_page/advanced_page.html">
 <link rel="import" href="/basic_page/basic_page.html">
 <link rel="import" href="/route.html">
 <link rel="import" href="/settings_page/main_page_behavior.html">
@@ skipping 60 matching lines @@
           on-subpage-expand="onSubpageExpand_"
           on-freeze-scroll="onFreezeScroll_">
       </settings-basic-page>
     </template>
     <template is="dom-if"
         if="[[showAdvancedSettings_(pageVisibility.advancedSettings)]]">
       <template is="dom-if" if="[[showAdvancedToggle_(
           showPages_.basic, hasExpandedSection_, inSearchMode_)]]">
         <div id="toggleSpacer"></div>
         <h2 id="toggleContainer">
-          <paper-button id="advancedToggle" on-tap="toggleAdvancedPage_">
+          <paper-button id="advancedToggle" on-tap="toggleAdvancedPage_" toggles
+              active="[[advancedToggleExpanded_]]"
+              aria-active-attribute="aria-expanded">
             <span>$i18n{advancedPageTitle}</span>
             <iron-icon icon="[[arrowState_(advancedToggleExpanded_)]]">
             </iron-icon>
           </paper-button>
         </h2>
       </template>
       <template is="dom-if" if="[[showAdvancedPage_(
           showPages_.advanced, inSearchMode_, hasExpandedSection_)]]">
         <settings-advanced-page prefs="{{prefs}}"
             page-visibility="[[pageVisibility]]"
             on-subpage-expand="onSubpageExpand_"
             on-freeze-scroll="onFreezeScroll_">
         </settings-advanced-page>
       </template>
     </template>
     <template is="dom-if" if="[[showPages_.about]]">
       <settings-about-page></settings-about-page>
     </template>
     <div id="overscroll" style="padding-bottom: [[overscroll_]]px"></div>
   </template>
   <script src="settings_main.js"></script>
   <script src="/search_settings.js"></script>
 </dom-module>