feat(window.loaded): Add new metric. (#14965)

index.html

@@ -69,6 +69,11 @@
 
         window.indexLoadedTime = window.performance.now();
         console.log("(TIME) index.html loaded:\t", indexLoadedTime);
+        window.addEventListener('load', function() {
+            window.loadedEventTime = window.performance.now();
+            console.log("(TIME) window loaded event:\t", loadedEventTime);
+        });
+
         // XXX the code below listeners for errors and displays an error message
         // in the document body when any of the required files fails to load.
         // The intention is to prevent from displaying broken page.

react/index.web.js

@@ -51,6 +51,10 @@ globalNS.connectionTimes = {
     'index.loaded': window.indexLoadedTime
 };
 
+window.addEventListener('load', () => {
+    globalNS.connectionTimes['window.loaded'] = window.loadedEventTime;
+});
+
 document.addEventListener('DOMContentLoaded', () => {
     const now = window.performance.now();