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

chrome/browser/resources/chromeos/quick_unlock/pin_keyboard.html

 <!-- TODO(crbug.com/603217): Use i18n instead of string literals. Figure out
                              what i18n to use for keypad, ie, does 1 ABC make
                              sense in every scenario? -->
 
 <link rel="import" href="chrome://resources/cr_elements/icons.html">
 <link rel="import" href="chrome://resources/cr_elements/shared_vars_css.html">
 <link rel="import" href="chrome://resources/html/i18n_behavior.html">
 <link rel="import" href="chrome://resources/html/polymer.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">
@@ skipping 165 matching lines @@
 
     <div id="root">
       <div id="container-constrained-width">
         <div class="row first-row">
           <paper-input id="pinInput" type="password" no-label-float
               value="[[value]]"
               is-input-rtl$="[[isInputRtl_(value)]]"
               has-content$="[[hasInput_(value)]]"
               label="[[getInputPlaceholder_(enablePassword)]]"
               on-keydown="onInputKeyDown_"
-              hidden="[[!showPinInput_]]">
+              hidden="[[!showPinInput_]]" autofocus>
           </paper-input>
         </div>
         <content select="[problem]"></content>
         <div class="separator" hidden="[[showPinInput_]]"></div>
         <div class="row keyboard">
           <paper-button class="digit-button" on-tap="onNumberTap_" value="1"
               noink>
             <inner-text>$i18n{pinKeyboard1}</inner-text>
             <paper-ripple>
           </paper-button>
@@ skipping 60 matching lines @@
                 noink>
             </paper-icon-button>
             <paper-ripple>
           </div>
         </div>
       </div>
     </div>
   </template>
   <script src="pin_keyboard.js"></script>
 </dom-module>