Reland: Explicitly track the scheduler task enqueueing order in a new field

components/scheduler/child/task_queue_manager.cc

Index: components/scheduler/child/task_queue_manager.cc
diff --git a/components/scheduler/child/task_queue_manager.cc b/components/scheduler/child/task_queue_manager.cc
index c557665ab7a4b6a5fac49b02fa6f58a461135f56..161c1dc3676311567c3de0cb70557e033530bfd3 100644
--- a/components/scheduler/child/task_queue_manager.cc
+++ b/components/scheduler/child/task_queue_manager.cc
@@ -104,7 +104,7 @@ void TaskQueueManager::UnregisterAsUpdatableTaskQueue(
 
 void TaskQueueManager::UpdateWorkQueues(
     bool should_trigger_wakeup,
-    const base::PendingTask* previous_task) {
+    const internal::SchedulerTask* previous_task) {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   internal::LazyNow lazy_now(this);
 
@@ -159,8 +159,7 @@ void TaskQueueManager::DoWork(bool posted_from_main_thread) {
   // pump-after-wakeup queue.
   UpdateWorkQueues(false, nullptr);
 
-  base::PendingTask previous_task((tracked_objects::Location()),
-                                  (base::Closure()));
+  internal::SchedulerTask previous_task;
   for (int i = 0; i < work_batch_size_; i++) {
     internal::TaskQueueImpl* queue;
     if (!SelectQueueToService(&queue))
@@ -192,16 +191,18 @@ bool TaskQueueManager::SelectQueueToService(
   return should_run;
 }
 
-void TaskQueueManager::DidQueueTask(const base::PendingTask& pending_task) {
+void TaskQueueManager::DidQueueTask(internal::SchedulerTask& pending_task) {
+  pending_task.sequence_num = GetNextSequenceNumber();
   task_annotator_.DidQueueTask("TaskQueueManager::PostTask", pending_task);
 }
 
 bool TaskQueueManager::ProcessTaskFromWorkQueue(
     internal::TaskQueueImpl* queue,
-    base::PendingTask* out_previous_task) {
+    internal::SchedulerTask* out_previous_task) {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   scoped_refptr<DeletionSentinel> protect(deletion_sentinel_);
-  base::PendingTask pending_task = queue->TakeTaskFromWorkQueue();
+  // TODO(alexclarke): consider std::move() when allowed.
+  internal::SchedulerTask pending_task = queue->TakeTaskFromWorkQueue();
 
   if (queue->GetQuiescenceMonitored())
     task_was_run_on_quiescence_monitored_queue_ = true;
@@ -286,6 +287,10 @@ int TaskQueueManager::GetNextSequenceNumber() {
   return task_sequence_num_.GetNext();
 }
 
+int TaskQueueManager::GetNextAgeNumber() {
+  return task_age_num_.GetNext();
+}
+
 scoped_refptr<base::trace_event::ConvertableToTraceFormat>
 TaskQueueManager::AsValueWithSelectorResult(
     bool should_run,