MD Settings: fix overflow scrolling for <settings-dialog>

chrome/browser/resources/settings/site_settings/add_site_dialog.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-button/paper-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://md-settings/i18n_setup.html">
 <link rel="import" href="chrome://md-settings/settings_dialog.html">
 <link rel="import" href="chrome://md-settings/settings_shared_css.html">
 
 <dom-module id="add-site-dialog">
   <template>
     <style include="settings-shared"></style>
     <settings-dialog id="dialog">
       <div class="title">$i18n{addSiteHeader}</div>
       <div class="body">
         <span class="start">$i18n{addSite}</span>
         <paper-input type="text" id="site" autofocus
             value="{{site_}}" on-input="validate_"></paper-input>
       </div>
-      <div class="button-container">
+      <div class="buttons">
         <paper-button class="cancel-button" dialog-dismiss>
             $i18n{cancel}
         </paper-button>
         <paper-button class="action-button" id="add"
             on-tap="onAddTap_" disabled>
           $i18n{add}
         </paper-button>
       </div>
     </settings-dialog>
   </template>
   <script src="add_site_dialog.js"></script>
 </dom-module>