Flakiness was caused by this test assuming the time move forward without ensuring so. (#41455)

Ensure the monotonic time moves forward by busy looping fo 2ms.
diff --git a/requestidlecallback/callback-idle-periods.html b/requestidlecallback/callback-idle-periods.html
index f515755..edf1adc 100644
--- a/requestidlecallback/callback-idle-periods.html
+++ b/requestidlecallback/callback-idle-periods.html
@@ -15,6 +15,7 @@
     var now = performance.now();
     var remaining = deadline.timeRemaining();
     var new_deadline = now + remaining;
+    while (now + 2 > performance.now());
     if (previous_deadline != undefined) {
       assert_true(new_deadline >= previous_deadline, "A requestIdleCallback scheduled during an idle period should be called back with a deadline greater than or equal to that in the current idle period.");
     }
@@ -23,6 +24,8 @@
     if (--idle_callbacks_remaining > 0) {
       previous_deadline = new_deadline;
       requestIdleCallback(rIC);
+      let now = performance.now();
+      while (now + 2 > performance.now());
     } else {
       this.done();
     }