Bug 1276490 - Do not use Math.random() to create unique values to avoid intermittent failrues. r=jgraham

MozReview-Commit-ID: 3QHrpCTpEzM

--HG--
extra : rebase_source : 7aaa4dae880b2e49fa0dd36d1eaba0d723d81592

testing/web-platform/tests/performance-timeline/po-resource.html

@@ -15,11 +15,15 @@ New resources will <a href="https://w3c.github.io/performance-timeline/#dfn-queu
       var filename = pathname.substring(pathname.lastIndexOf('/')+1);
       return pathname.substring(0, pathname.length - filename.length);
     }
+    var gUniqueCounter = 0;
+    function generateUniqueValues() {
+      return Date.now() + "-" + (++gUniqueCounter);
+    }
     var stored_entries = [];
     var img_location = document.location.origin + path(document.location.pathname)
        + "resources/square.png?random=";
-    var img1 = img_location + Math.floor(Math.random() * 100);
-    var img2 = img_location + Math.floor(Math.random() * 1000);
+    var img1 = img_location + generateUniqueValues();
+    var img2 = img_location + generateUniqueValues();
     var observer = new PerformanceObserver(
         t.step_func(function (entryList, obs) {
           stored_entries =