Add a histogram for tracking web store promo

chrome/browser/resources/new_new_tab.js

 // Copyright (c) 2010 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.
 
 // To avoid creating tons of unnecessary nodes. We assume we cannot fit more
 // than this many items in the miniview.
 var MAX_MINIVIEW_ITEMS = 15;
 
 // Extra spacing at the top of the layout.
 var LAYOUT_SPACING_TOP = 25;
@@ skipping 1188 matching lines @@
 }
 
 function isDoneLoading() {
   return !document.body.classList.contains('loading');
 }
 
 // Initialize the apps promo.
 document.addEventListener('DOMContentLoaded', function() {
   var promoText1 = $('apps-promo-text1');
   promoText1.innerHTML = promoText1.textContent;
-  promoText1.querySelector('a').href = localStrings.getString('web_store_url');
+
+  var promoLink = promoText1.querySelector('a');
+  promoLink.id = 'apps-promo-link';
+  promoLink.href = localStrings.getString('web_store_url');
 
   $('apps-promo-hide').addEventListener('click', function() {
     chrome.send('hideAppsPromo', []);
     document.documentElement.classList.remove('apps-promo-visible');
     layoutSections();
   });
 });