MD Settings: Style native <select> down arrow.

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

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="/controls/pref_control_behavior.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/md_select_css.html">
 <link rel="import" href="/prefs/pref_util.html">
 <link rel="import" href="/settings_shared_css.html">
 
 <dom-module id="settings-dropdown-menu">
   <template>
     <style include="settings-shared md-select"></style>
-    <div class="select-wrapper">
-      <select id="dropdownMenu" on-change="onChange_" value="[[selected_]]"
+    <div class="md-select-wrapper">
+      <select class="md-select" id="dropdownMenu" on-change="onChange_"
+          value="[[selected_]]"
           disabled="[[shouldDisableMenu_(disabled, menuOptions)]]">
         <template is="dom-repeat" items="[[menuOptions]]">
           <option value="[[item.value]]" data-value$="[[item.value]]">
             [[item.name]]
           </option>
         </template>
         <option value="[[notFoundValue_]]"
             hidden$="[[!isSelectedNotFound_(selected_)]]">
           $i18n{custom}
         </option>
       </select>
-      <span class="select-underline"></span>
+      <span class="md-select-underline"></span>
     </div>
   </template>
   <script src="settings_dropdown_menu.js"></script>
 </dom-module>