MD Settings: disable the On Startup "OK" button with no site content

chrome/browser/resources/settings/on_startup_page/startup_urls_page.html

 <link rel="import" href="chrome://resources/polymer/v1_0/polymer/polymer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-dialog/paper-dialog.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-input/paper-input.html">
 <link rel="import" href="chrome://md-settings/settings_dialog_css.html">
 <link rel="import" href="chrome://md-settings/settings_shared_css.html">
 
 <dom-module id="settings-startup-urls-page">
   <link rel="import" type="css" href="on_startup_shared.css">
   <template>
     <style include="settings-dialog"></style>
@@ skipping 21 matching lines @@
     <paper-dialog modal id="addUrlDialog" class="layout vertical">
       <div id="dialog-content">
         <div class="top-row">
           <div class="title" i18n-content="onStartupAddNewPage"></div>
           <paper-icon-button icon="clear" on-tap="onCancelTap_" id="close">
           </paper-icon-button>
         </div>
         <div class="body">
           <div class="explanation">
             <paper-input class="flex" always-float-label
-                i18n-values="label:onStartupSiteUrl" value="{{newUrl}}">
+                i18n-values="label:onStartupSiteUrl" value="{{newUrl_}}">
             </paper-input>
           </div>
           <div class="button-container">
             <div class="action-buttons">
               <paper-button class="cancel-button" on-tap="onCancelTap_"
                   id="cancel" i18n-content="cancel"></paper-button>
               <!-- TODO(dbeam): this button should be [disabled] when |newUrl|
                    is empty. -->
               <paper-button class="action-button" on-tap="onOkTap_"
                   id="addUrlButton" i18n-content="add"></paper-button>
             </div>
           </div>
         </div>
       </div>
     </paper-dialog>
 
   </template>
   <script src="startup_urls_page.js"></script>
 </dom-module>