MD Settings: Autofocus first text input in various dialogs.

chrome/browser/resources/settings/site_settings/edit_exception_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="../i18n_setup.html">
 <link rel="import" href="site_settings_prefs_browser_proxy.html">
 
 <dom-module id="settings-edit-exception-dialog">
   <template>
     <style include="settings-shared"></style>
     <dialog is="cr-dialog" id="dialog">
       <div class="title">$i18n{editSiteTitle}</div>
       <div class="body">
         <paper-input always-float-label label="$i18n{addSite}"
             placeholder="$i18n{addSiteExceptionPlaceholder}"
-            value="{{origin_}}" on-input="validate_" on-keypress="onKeypress_" >
+            value="{{origin_}}" on-input="validate_" on-keypress="onKeypress_"
+            autofocus>
         </paper-input>
       </div>
       <div class="button-container">
         <div class="action-buttons">
           <paper-button class="cancel-button" on-tap="onCancelTap_"
               id="cancel">$i18n{cancel}</paper-button>
           <paper-button id="actionButton" class="action-button"
               on-tap="onActionButtonTap_">$i18n{edit}</paper-button>
         </div>
       </div>
     </dialog>
   </template>
   <script src="edit_exception_dialog.js"></script>
 </dom-module>