[ThreadPool]: Use sequence num to compare tasks in DelayedTaskManager.

To ensure re-ordering of delayed tasks posted at the same time, sequence_num
is used to give priority to the task that was first created.

Change-Id: I97f429d23991259af43395d229cb76cb3fc0e93c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1778926
Commit-Queue: Etienne Pierre-Doray <etiennep@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#692246}

base/task/thread_pool/delayed_task_manager.cc

@@ -35,7 +35,10 @@ DelayedTaskManager::DelayedTask& DelayedTaskManager::DelayedTask::operator=(
 
 bool DelayedTaskManager::DelayedTask::operator<=(
     const DelayedTask& other) const {
-  return task.delayed_run_time <= other.task.delayed_run_time;
+  if (task.delayed_run_time == other.task.delayed_run_time) {
+    return task.sequence_num <= other.task.sequence_num;
+  }
+  return task.delayed_run_time < other.task.delayed_run_time;
 }
 
 bool DelayedTaskManager::DelayedTask::IsScheduled() const {