Account for the case when there are no resource timings

tools/perf/metrics/speedindex.js

 // Copyright 2013 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.
 
 'use strict';
 
 /**
  * @fileoverview This file provides a JavaScript helper function that
  * determines when the last resource was received.
  */
@@ skipping 26 matching lines @@
   // new entries are not added. We're only interested in the last entry,
   // so we can clear new entries when they're added.
   var lastEntry = null;
 
   /**
    * This method uses the Resource Timing interface, which is described at
    * http://www.w3.org/TR/resource-timing/. It provides information about
    * timings for resources such as images and script files, and it includes
    * resources requested via XMLHttpRequest.
    *
-   *
    * @return {number} The time since either the load event, or the last
    *   the last resource was received after the load event. If the load
    *   event hasn't yet happened, return 0.
    */
   window.timeSinceLastResponseAfterLoadMs = function() {
     if (window.document.readyState !== 'complete') {
       return 0;
     }
 
     var resourceTimings = window.performance.getEntriesByType('resource');
     if (resourceTimings.length > 0) {
       lastEntry = resourceTimings.pop();
       window.performance.clearResourceTimings();
     }
 
     // The times for performance.now() and in the PerformanceResourceTiming
     // objects are all in milliseconds since performance.timing.navigationStart,
     // so we must also get load time in the same terms.
     var timing = window.performance.timing;
     var loadTime = timing.loadEventEnd - timing.navigationStart;
-    if (loadTime > lastEntry.responseEnd) {
+
+    // If there have been no resource timing entries, or the last entry was
+    // before the load event, then return the time since the load event.
+    if (!lastEntry || lastEntry.responseEnd < loadTime) {
       return window.performance.now() - loadTime;
     }
     return window.performance.now() - lastEntry.responseEnd;
   }
 
 })();