ntp4: disable animations for pages that are off screen

chrome/browser/resources/ntp4/card_slider.js

Index: chrome/browser/resources/ntp4/card_slider.js
diff --git a/chrome/browser/resources/ntp4/card_slider.js b/chrome/browser/resources/ntp4/card_slider.js
index 0cd671119532bca94149892b8c09b87cd8354848..ca18f4e90ae2304197ebc0a50263abdc9ae122ee 100644
--- a/chrome/browser/resources/ntp4/card_slider.js
+++ b/chrome/browser/resources/ntp4/card_slider.js
@@ -263,13 +263,15 @@ var CardSlider = (function() {
      *     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);