Implemented event-dispatching for button clicks.

ui/login/screen.js

Index: ui/login/screen.js
diff --git a/ui/login/screen.js b/ui/login/screen.js
index 93efd73e2455b7acac1b62e70e860871ed0218dd..dc8849c9a33877074b144f8735a1078f609541be 100644
--- a/ui/login/screen.js
+++ b/ui/login/screen.js
@@ -6,8 +6,9 @@
  * @fileoverview Base class for all login WebUI screens.
  */
 cr.define('login', function() {
-  /** @const */ var CALLBACK_USER_ACTED = 'userActed';
+  /** @const */ var CALLBACK_BUTTON_CLICKED = 'buttonClicked';
   /** @const */ var CALLBACK_CONTEXT_CHANGED = 'contextChanged';
+  /** @const */ var CALLBACK_USER_ACTED = 'userActed';
 
   function doNothing() {};
 
@@ -97,6 +98,22 @@ cr.define('login', function() {
     },
 
     /**
+     * @final
+     */
+    declareButton: function(id) {
+      var self = this;
+      var button = this.ownerDocument.createElement('button');
+      button.id = id;
+
+      button.addEventListener('click', function(e) {
+        self.sendImpl_(CALLBACK_BUTTON_CLICKED, id);
+        e.stopPropagation();
+      });
+
+      return button;
+    },
+
+    /**
      * @override
      * @final
      */