MD Settings: use the subLabel of <settings-toggle-button>s for the aria

chrome/browser/resources/settings/controls/settings_toggle_button.html

 <link rel="import" href="chrome://resources/cr_elements/policy/cr_policy_pref_indicator.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-toggle-button/paper-toggle-button.html">
 <link rel="import" href="settings_boolean_control_behavior.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-toggle-button">
   <template>
     <style include="settings-shared">
       :host([elide-label]),
@@ skipping 21 matching lines @@
       ::content .more-actions,
       cr-policy-pref-indicator {
         -webkit-margin-start: var(--settings-control-spacing);
       }
     </style>
     <div id="outerRow" noSubLabel$="[[!subLabel]]">
       <div class="flex" id="labelWrapper" on-tap="onLabelWrapperTap_"
           actionable$="[[!controlDisabled_(disabled, pref.*)]]"
           hidden="[[!label]]">
         <div id="label" class="label">[[label]]</div>
-        <div class="secondary label">[[subLabel]]</div>
+        <div id="subLabel" class="secondary label">[[subLabel]]</div>
       </div>
       <content select=".more-actions"></content>
       <template is="dom-if" if="[[hasPrefPolicyIndicator(pref.*)]]">
         <cr-policy-pref-indicator pref="[[pref]]" icon-aria-label="[[label]]">
         </cr-policy-pref-indicator>
       </template>
       <paper-toggle-button id="control" checked="{{checked}}"
           on-change="notifyChangedByUserInteraction" aria-labelledby="label"
+          aria-describedby="subLabel"
           disabled="[[controlDisabled_(disabled, pref)]]">
       </paper-toggle-button>
     </div>
   </template>
   <script src="settings_toggle_button.js"></script>
 </dom-module>