[NTP4] Make TilePage and CardSlider evented to simplify code and fix page switcher bug

chrome/browser/resources/ntp4/page_switcher.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Page switcher
  * This is the class for the left and right navigation arrows that switch
  * between pages.
  */
 cr.define('ntp4', function() {
@@ skipping 14 matching lines @@
 
       el.dragWrapper_ = new cr.ui.DragWrapper(el, el);
     },
 
     /**
      * Activate the switcher (go to the next card).
      * @private
      */
     activate_: function() {
       var cardSlider = ntp4.getCardSlider();
-      cardSlider.selectCard(cardSlider.currentCard + this.direction_, true);
+      var index = cardSlider.currentCard + this.direction_;
+      var numCards = cardSlider.cardCount - 1;
+      cardSlider.selectCard(Math.max(0, Math.min(index, numCards)), true);
     },
 
     /**
      * Handler for the mousewheel event on a pager. We pass through the scroll
      * to the page. This is necssary because the page is our sibling in the DOM
      * hierarchy, so the event won't naturally pass through to it.
      * @param {Event} e The mousewheel event.
      * @private
      */
     onMouseWheel_: function(e) {
@@ skipping 63 matching lines @@
         this.dragNavTimeout_ = null;
       }
     },
 
   };
 
   return {
     initializePageSwitcher: PageSwitcher.template.decorate
   }
 });