Add a very basic PIN UI implementation that is shared between lock and settings.

ui/login/account_picker/user_pod_row.js

Index: ui/login/account_picker/user_pod_row.js
diff --git a/ui/login/account_picker/user_pod_row.js b/ui/login/account_picker/user_pod_row.js
index 26bd8cadee24f51e8c299a79ea56b206269c52dc..95413d4a6fe6b720dbab768a27810883b2b1bd90 100644
--- a/ui/login/account_picker/user_pod_row.js
+++ b/ui/login/account_picker/user_pod_row.js
@@ -732,6 +732,10 @@ cr.define('login', function() {
             'keydown',
             this.handleRemoveUserConfirmationKeyDown_.bind(this));
 
+      var pinKeyboard = document.getElementById('pin-keyboard');
+      pinKeyboard.addEventListener('submit',
+          this.handlePinSubmitted_.bind(this));
+
       var customIcon = this.customIconElement;
       customIcon.parentNode.replaceChild(new UserPodCustomIcon(), customIcon);
     },
@@ -765,6 +769,16 @@ cr.define('login', function() {
     },
 
     /**
+     * Handles the user hitting 'submit' on the PIN keyboard.
+     * @param {Event} e Submit event object.
+     * @private
+     */
+    handlePinSubmitted_: function(e) {
+      var pin = e.detail.pin;
+      chrome.send('authenticateUserWithPin', [this.user.username, pin]);
+    },
+
+    /**
      * Handles keypress event (i.e. any textual input) on password input.
      * @param {Event} e Keypress Event object.
      * @private