ntp4: disable animations for pages that are off screen

chrome/browser/resources/ntp4/card_slider.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 Card slider implementation. Allows you to create interactions
  * that have items that can slide left to right to reveal additional items.
  * Works by adding the necessary event handlers to a specific DOM structure
  * including a frame, container and cards.
  * - The frame defines the boundary of one item. Each card will be expanded to
@@ skipping 245 matching lines @@
     },
 
     /**
      * Selects a new card, ensuring that it is a valid index, transforming the
      * view and possibly calling the change card callback.
      * @param {number} newCardIndex Index of card to show.
      * @param {boolean=} opt_animate If true will animate transition from
      *     current position to new position.
      */
     selectCard: function(newCardIndex, opt_animate) {
-      var isChangingCard = newCardIndex >= 0 &&
-          newCardIndex < this.cards_.length &&
-          newCardIndex != this.currentCard;
+      var isChangingCard =
+          !this.cards_[newCardIndex].classList.contains('selected-card');
+
       if (isChangingCard) {
+        this.currentCardValue.classList.remove('selected-card');
         // If we have a new card index and it is valid then update the left
         // position and current card index.
         this.currentCard_ = newCardIndex;
+        this.currentCardValue.classList.add('selected-card');
       }
 
       this.transformToCurrentCard_(opt_animate);
 
       if (isChangingCard) {
         var event = document.createEvent('Event');
         event.initEvent(CardSlider.EventType.CARD_CHANGED, true, true);
         event.cardSlider = this;
         this.container_.dispatchEvent(event);
       }
@@ skipping 117 matching lines @@
       // Stop listening to any current touch
       this.touchHandler_.cancelTouch();
 
       // Ensure we're at a card bounary
       this.transformToCurrentCard_(true);
     },
   };
 
   return CardSlider;
 })();