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

components/scheduler/child/task_queue_manager.cc

 // Copyright 2014 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.
 
 #include "components/scheduler/child/task_queue_manager.h"
 
 #include <queue>
 #include <set>
 
 #include "base/bind.h"
@@ skipping 107 matching lines @@
   }
 
   auto iter = updatable_queue_set_.begin();
   while (iter != updatable_queue_set_.end()) {
     internal::TaskQueueImpl* queue = *iter++;
     // NOTE Update work queue may erase itself from |updatable_queue_set_|.
     // This is fine, erasing an element won't invalidate any interator, as long
     // as the iterator isn't the element being delated.
     if (queue->work_queue().empty())
         queue->UpdateWorkQueue(&lazy_now, should_trigger_wakeup, previous_task);
-    if (!queue->work_queue().empty()) {
-      // Currently we should not be getting tasks with delayed run times in any
-      // of the work queues.
-      DCHECK(queue->work_queue().front().delayed_run_time.is_null());
-    }
   }
 }
 
 void TaskQueueManager::MaybePostDoWorkOnMainRunner() {
   bool on_main_thread = main_task_runner_->BelongsToCurrentThread();
   if (on_main_thread) {
     // We only want one pending DoWork posted from the main thread, or we risk
     // an explosion of pending DoWorks which could starve out everything else.
     if (pending_dowork_count_ > 0) {
       return;
@@ skipping 51 matching lines @@
 
 void TaskQueueManager::DidQueueTask(const base::PendingTask& pending_task) {
   task_annotator_.DidQueueTask("TaskQueueManager::PostTask", pending_task);
 }
 
 bool TaskQueueManager::ProcessTaskFromWorkQueue(
     internal::TaskQueueImpl* queue,
     base::PendingTask* 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::TaskQueueImpl::Task pending_task = queue->TakeTaskFromWorkQueue();
 
   if (queue->GetQuiescenceMonitored())
     task_was_run_on_quiescence_monitored_queue_ = true;
 
   if (!pending_task.nestable && main_task_runner_->IsNested()) {
     // Defer non-nestable work to the main task runner.  NOTE these tasks can be
     // arbitrarily delayed so the additional delay should not be a problem.
     main_task_runner_->PostNonNestableTask(pending_task.posted_from,
                                            pending_task.task);
   } else {
@@ skipping 94 matching lines @@
   state->EndArray();
   return state;
 }
 
 void TaskQueueManager::OnTaskQueueEnabled() {
   DCHECK(main_thread_checker_.CalledOnValidThread());
   MaybePostDoWorkOnMainRunner();
 }
 
 }  // namespace scheduler