MD Settings: Prevent <settings-prefs> from being initialized in guest mode.

chrome/browser/resources/settings/settings_ui/settings_ui.html

 <link rel="import" href="chrome://resources/cr_elements/cr_toolbar/cr_toolbar.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/polymer/v1_0/app-layout/app-drawer/app-drawer.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/iron-flex-layout/iron-flex-layout.html">
 <link rel="import" href="chrome://resources/polymer/v1_0/paper-header-panel/paper-header-panel.html">
 <link rel="import" href="/direction_delegate.html">
 <link rel="import" href="/i18n_setup.html">
 <link rel="import" href="/icons.html">
 <link rel="import" href="/settings_main/settings_main.html">
@@ skipping 66 matching lines @@
       app-drawer .drawer-header {
         -webkit-padding-start: 24px;
         border-bottom: var(--settings-separator-line);
       }
 
       app-drawer .drawer-content {
         height: calc(100% - 56px);
         overflow: auto;
       }
     </style>
-    <settings-prefs id="prefs" prefs="{{prefs}}"></settings-prefs>
+    <template is="dom-if" if="[[!isGuestMode_]]">
+      <settings-prefs id="prefs" prefs="{{prefs}}"></settings-prefs>
+    </template>
     <cr-toolbar page-name="$i18n{settings}" clear-label="$i18n{clearSearch}"
         search-prompt="$i18n{searchPrompt}" on-cr-menu-tap="onMenuButtonTap_"
         spinner-active="[[toolbarSpinnerActive_]]" show-menu
         menu-label="$i18n{menuButtonLabel}">
     </cr-toolbar>
     <app-drawer swipe-open>
       <div class="drawer-header">$i18n{settings}</div>
       <div class="drawer-content">
         <template is="dom-if" id="drawerTemplate">
           <settings-menu page-visibility="[[pageVisibility_]]"
               on-iron-activate="onIronActivate_">
           </settings-menu>
         </template>
       </div>
     </app-drawer>
     <paper-header-panel mode="waterfall">
       <settings-main prefs="{{prefs}}"
           toolbar-spinner-active="{{toolbarSpinnerActive_}}"
           page-visibility="[[pageVisibility_]]">
       </settings-main>
     </paper-header-panel>
   </template>
   <script src="settings_ui.js"></script>
 </dom-module>