md-settings: wrap lock screen dialogs in dom-if.

chrome/browser/resources/settings/people_page/password_prompt_dialog.html

 <link rel="import" href="chrome://resources/html/polymer.html">
 
 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.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-input/paper-input.html">
 <link rel="import" href="lock_screen_constants.html">
 <link rel="import" href="../settings_shared_css.html">
 
 <dom-module id="settings-password-prompt-dialog">
   <template>
     <style include="settings-shared">
       #passwordInput {
         display: inline-block;
       }
 
       .settings-box {
         padding: 0;
       }
     </style>
 
     <dialog is="cr-dialog" id="dialog" on-close="onClose_"
         close-text="$i18n{close}">
       <div class="title">$i18n{passwordPromptTitle}</div>
       <div class="body">
 
         <div class="settings-box first">$i18n{passwordPromptEnterPassword}</div>
 
-        <paper-input id="passwordInput" type="password" autofocus
+        <paper-input id="passwordInput" type="password"
             label="$i18n{passwordPromptPasswordLabel}"
             value="{{password_}}"
             invalid$="[[passwordInvalid_]]"
             error-message="$i18n{passwordPromptInvalidPassword}"
             aria-disabled="false">
         </paper-input>
       </div>
       <div class="button-container">
         <paper-button class="cancel-button" on-tap="onCancelTap_">
           $i18n{cancel}
         </paper-button>
 
         <paper-button class="action-button" on-tap="submitPassword_"
             disabled$="[[!enableConfirm_(password_, passwordInvalid_)]]">
           $i18n{confirm}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="password_prompt_dialog.js"></script>
 </dom-module>