MD-Settings: A11y - Label power source and stylus drop downs.

chrome/browser/resources/settings/device_page/power.html

 <link rel="import" href="chrome://resources/html/cr.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/md_select_css.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="../route.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-power">
   <template>
     <style include="settings-shared md-select"></style>
     <div id="powerSourceRow" class="settings-box first two-line">
       <div class="start">
         <div>[[powerSourceLabel_]]</div>
         <div class="secondary">[[batteryStatus_.statusText]]</div>
       </div>
       <div class="md-select-wrapper" hidden$="[[!showPowerSourceDropdown_]]">
         <select id="powerSource" class="md-select"
+            aria-label="$i18n{powerSourceLabel}"
             on-change="onPowerSourceChange_">
           <option value="" selected="[[isEqual_('', selectedPowerSourceId_)]]">
             $i18n{powerSourceBattery}
           </option>
           <template is="dom-repeat" items="[[powerSources_]]">
             <option value="[[item.id]]"
                 selected="[[isEqual_(item.id, selectedPowerSourceId_)]]">
               [[item.description]]
             </option>
           </template>
         </select>
         <span class="md-select-underline"></span>
       </div>
       <div hidden$="[[showPowerSourceDropdown_]]">
         [[powerSourceName_]]
       </div>
     </div>
   </template>
   <script src="power.js"></script>
 </dom-module>