Add a histogram for tracking web store promo

chrome/browser/resources/new_new_tab.js

Index: chrome/browser/resources/new_new_tab.js
diff --git a/chrome/browser/resources/new_new_tab.js b/chrome/browser/resources/new_new_tab.js
index 6b3be4ad5560bb9e2ead69d5f11da662574cb817..89deeeff8a30f4927a6d985efb22f65605e83b3c 100644
--- a/chrome/browser/resources/new_new_tab.js
+++ b/chrome/browser/resources/new_new_tab.js
@@ -1206,7 +1206,10 @@ function isDoneLoading() {
 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', []);