Prevent Add Exception from showing on the All Sites list.

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

 <link rel="import" href="chrome://resources/cr_elements/icons.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="chrome://resources/polymer/v1_0/iron-icon/iron-icon.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-icon-button/paper-icon-button.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-menu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu/paper-submenu.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-menu-button/paper-menu-button.html">
 <link rel="import" href="/i18n_setup.html">
@@ skipping 64 matching lines @@
                 </paper-item>
                 <paper-item
                     menu-value$="[[PermissionValues.DEFAULT]]">
                   $i18n{siteSettingsActionReset}
                 </paper-item>
               </paper-menu>
             </paper-menu-button>
           </div>
         </template>
 
-        <div class="button-container">
+        <div class="button-container" hidden$="[[allSites]]">

[Dan Beam, 2016/07/06 16:23:27]

nit: it may be worthwhile to use dom-if instead of hidden here because
<paper-button> has a lot of elements used to implement it (it's heavier than you
think)

[Finnur, 2016/07/07 09:50:10]

Fair enough. Done. Checking in.

           <paper-button on-tap="onAddSiteTap_" class="primary-button no-upper">
             $i18n{addSiteLink}
           </paper-button>
         </div>
       </div>
     </paper-submenu>
   </template>
   <script src="site_list.js"></script>
 </dom-module>