Site Settings Desktop: Flesh out controlled exceptions a little more.

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

 <link rel="import" href="chrome://resources/html/polymer.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-dropdown-menu/paper-dropdown-menu-light.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-listbox/paper-listbox.html">
 <link rel="import" href="chrome://resources/html/web_ui_listener_behavior.html">
 <link rel="import" href="/settings_shared_css.html">
 <link rel="import" href="/site_settings/constants.html">
 <link rel="import" href="/site_settings/site_settings_behavior.html">
 <link rel="import" href="/site_settings/site_settings_prefs_browser_proxy.html">
 
 <dom-module id="site-details-permission">
   <template>
     <style include="settings-shared">
     </style>
     <div id="details" hidden>
       <div class="list-item underbar">
         <div>
           <iron-icon icon="[[computeIconForContentCategory(category)]]">
           </iron-icon>
         </div>
         <div class="middle" id="permissionHeader">
           [[computeTitleForContentCategory(category)]]
         </div>
-        <paper-dropdown-menu-light vertical-align="auto" no-label-float>
+        <paper-dropdown-menu-light vertical-align="auto" no-label-float
+            disabled$="[[isExceptionControlled_(site.source)]]">
           <paper-listbox id="permission" class="dropdown-content"
               on-iron-activate="onPermissionMenuIronActivate_"
               attr-for-selected="data-permission-value">
             <button id="allow" class="dropdown-item" role="option"
                 data-permission-value$="[[PermissionValues.ALLOW]]">
               $i18n{siteSettingsActionAllow}
             </button>
             <button id="block" class="dropdown-item" role="option"
                 data-permission-value$="[[PermissionValues.BLOCK]]">
               $i18n{siteSettingsActionBlock}
             </button>
           </paper-listbox>
         </paper-dropdown-menu-light>
       </div>
     </div>
   </template>
   <script src="site_details_permission.js"></script>
 </dom-module>