MD Settings: Listen to the Enter key in the Add User ChromeOS dialog.

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

 <link rel="import" href="chrome://resources/cr_elements/cr_dialog/cr_dialog.html">
 <link rel="import" href="chrome://resources/html/assert.html">
 <link rel="import" href="chrome://resources/html/polymer.html">
+<link rel="import" href="chrome://resources/polymer/v1_0/iron-a11y-keys/iron-a11y-keys.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="/settings_shared_css.html">
 
 <dom-module id="settings-users-add-user-dialog">
   <template>
     <style include="settings-shared">
       paper-input {
         width: var(--paper-input-max-width);
         --paper-input-container-focus-color: var(--google-blue-500);
         --paper-input-container-input: {
           font-size: inherit;
         };
       }
     </style>
     <dialog is="cr-dialog" id="dialog">
       <div class="title">$i18n{addUsers}</div>
       <div class="body">
         <paper-input id="addUserInput" label="$i18n{addUsersEmail}" autofocus
             on-input="validate_">
+          <iron-a11y-keys id="keys" keys="enter" on-keys-pressed="addUser_">
+          </iron-a11y-keys>
         </paper-input>
       </div>
       <div class="button-container">
         <paper-button class="cancel-button" on-tap="onCancelTap_"
             id="cancel">$i18n{cancel}</paper-button>
         <paper-button id="add" on-tap="addUser_" class="action-button" disabled>
           $i18n{add}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="users_add_user_dialog.js"></script>
 </dom-module>