MD Settings: add a11y secondary text (description) for Midi Devices in Content Settings

chrome/browser/resources/settings/site_settings_page/site_settings_page.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-icon-button/paper-icon-button-light.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="../settings_shared_css.html">
 <link rel="import" href="../site_settings/constants.html">
 <link rel="import" href="../site_settings/site_settings_behavior.html">
 <link rel="import" href="../site_settings/site_settings_prefs_browser_proxy.html">
 
@@ skipping 255 matching lines @@
       <div class="middle">
         $i18n{siteSettingsMidiDevices}
         <div class="secondary" id="midiDevicesSecondary">
           [[defaultSettingLabel_(
               default_.midiSysex,
               '$i18nPolymer{siteSettingsMidiDevicesAsk}',
               '$i18nPolymer{siteSettingsMidiDevicesBlock}')]]
         </div>
       </div>
       <button class="subpage-arrow" is="paper-icon-button-light"
-          aria-label="$i18n{siteSettingsMidiDevices}"></button>
+          aria-label="$i18n{siteSettingsMidiDevices}"
+          aria-describedby="midiDevicesSecondary"></button>
     </div>
     <div class="settings-box" category$="[[ContentSettingsTypes.ZOOM_LEVELS]]"
         data-route="SITE_SETTINGS_ZOOM_LEVELS"
         on-tap="onTapNavigate_" actionable>
       <iron-icon icon="settings:zoom-in"></iron-icon>
       <div class="middle">$i18n{siteSettingsZoomLevels}</div>
       <button class="subpage-arrow" is="paper-icon-button-light"
           aria-label="$i18n{siteSettingsZoomLevels}"></button>
     </div>
     <div class="settings-box" category$="[[ContentSettingsTypes.USB_DEVICES]]"
@@ skipping 14 matching lines @@
     <div class="settings-box" data-route="SITE_SETTINGS_PROTECTED_CONTENT"
         on-tap="onTapNavigate_" actionable>
       <iron-icon icon="settings:security"></iron-icon>
       <div class="middle">$i18n{siteSettingsProtectedContent}</div>
       <button class="subpage-arrow" is="paper-icon-button-light"
           aria-label="$i18n{siteSettingsProtectedContent}"></button>
     </div>
   </template>
   <script src="site_settings_page.js"></script>
 </dom-module>