MD Settings: For some content types, make per-site exceptions read-only.

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

 <link rel="import" href="chrome://resources/cr_elements/cr_action_menu/cr_action_menu.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-icon-button/paper-icon-button.html">
 <link rel="import" href="../i18n_setup.html">
 <link rel="import" href="../icons.html">
 <link rel="import" href="../route.html">
 <link rel="import" href="../settings_shared_css.html">
@@ skipping 12 matching lines @@
       }
 
       .selectable {
         -webkit-user-select: text;
       }
     </style>
     <div id="category">
       <div class="settings-box first" hidden$="[[allSites]]">
         <h2 class="start">[[categoryHeader]]</h2>
         <paper-button class="secondary-button header-aligned-button" id="icon"
-            on-tap="onAddSiteTap_">
+            hidden="[[readOnlyList]]" on-tap="onAddSiteTap_">
           $i18n{add}
         </paper-button>
       </div>
 
       <dialog is="cr-action-menu">
         <button class="dropdown-item" role="option" id="allow"
             on-tap="onAllowTap_" hidden$="[[!showAllowAction_]]">
           $i18n{siteSettingsActionAllow}
         </button>
         <button class="dropdown-item" role="option" id="block"
@@ skipping 30 matching lines @@
               <!-- This div must not contain extra whitespace. -->
               <div class="selectable secondary text-elide" id="siteDescription"
                   >[[computeSiteDescription_(item)]]</div>
             </div>
 
             <template is="dom-if" if="[[!!computeIconControlledBy_(item)]]">
               <iron-icon icon="[[computeIconControlledBy_(item)]]"></iron-icon>
             </template>
 
             <paper-icon-button id="dots" icon="cr:more-vert"
-                hidden="[[isActionMenuHidden_(item.source)]]"
+                hidden="[[isActionMenuHidden_(item.source, readOnlyList)]]"
                 on-tap="onShowActionMenuTap_" title="$i18n{moreActions}">
             </paper-icon-button>
             <template is="dom-if" if="[[enableSiteSettings_]]">
               <div on-tap="onOriginTap_" actionable>
                 <button class="subpage-arrow" is="paper-icon-button-light"
                     aria-label$="[[item.displayName]]"
                     aria-describedby="siteDescription"></button>
               </div>
             </template>
           </div>
         </template>
       </div>
     </div>
     <template is="dom-if" if="[[showEditExceptionDialog_]]" restamp>
       <settings-edit-exception-dialog model="[[actionMenuSite_]]"
           on-close="onEditExceptionDialogClosed_">
       </settings-edit-exception-dialog>
     </template>
   </template>
   <script src="site_list.js"></script>
 </dom-module>