MD Settings: Users: Fix add person validation and dialog focus

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/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" close-text="$i18n{close}">
       <div class="title">$i18n{addUsers}</div>
       <div class="body">
         <paper-input id="addUserInput" label="$i18n{addUsersEmail}" autofocus
-            on-input="validate_">
+            on-input="validate_" invalid="[[!isValid_]]">
         </paper-input>
       </div>
       <div class="button-container">
-        <paper-button class="cancel-button" on-tap="onCancelTap_"
-            id="cancel">$i18n{cancel}</paper-button>
-        <paper-button id="add" on-tap="addUser_" class="action-button" disabled>
+        <paper-button class="cancel-button" on-tap="onCancelTap_">
+          $i18n{cancel}
+        </paper-button>
+        <paper-button on-tap="addUser_" class="action-button"
+            disabled$="[[!isValid_]]">

[dpapad, 2017/04/11 00:03:55]

Probably "$=" not needed here, "=" should be sufficient.

[stevenjb, 2017/04/11 00:44:22]

Either works, Polymer maps it to an attribute, and we use a mix in Settings. I
guess the advantage of $ is that it can be used as an attribute in CSS also.

           $i18n{add}
         </paper-button>
       </div>
     </dialog>
   </template>
   <script src="users_add_user_dialog.js"></script>
 </dom-module>