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

components/scheduler/child/task_queue_impl.h

Index: components/scheduler/child/task_queue_impl.h
diff --git a/components/scheduler/child/task_queue_impl.h b/components/scheduler/child/task_queue_impl.h
index 1e05e5ab0069ef0dbc0198ca11b332e5ec8beee7..99c565b43e234f5885221bc49aae6da8cf64037b 100644
--- a/components/scheduler/child/task_queue_impl.h
+++ b/components/scheduler/child/task_queue_impl.h
@@ -27,6 +27,39 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
                 const char* disabled_by_default_tracing_category,
                 const char* disabled_by_default_verbose_tracing_category);
 
+  class SCHEDULER_EXPORT Task : public base::PendingTask {
+   public:
+    Task();
+    Task(const tracked_objects::Location& posted_from,
+         const base::Closure& task,
+         int sequence_number,
+         bool nestable);
+
+    int enqueue_order() const {
+#ifndef NDEBUG
+      DCHECK(enqueue_order_set_);
+#endif
+      return enqueue_order_;
+    }
+
+    void set_enqueue_order(int enqueue_order) {
+#ifndef NDEBUG
+      DCHECK(!enqueue_order_set_);
+      enqueue_order_set_ = true;
+#endif
+      enqueue_order_ = enqueue_order;
+    }
+
+   private:
+#ifndef NDEBUG
+    bool enqueue_order_set_;
+#endif
+    // Similar to sequence number, but the |enqueue_order| is set by
+    // EnqueueTasksLocked and is not initially defined for delayed tasks until
+    // they are enqueued on the |incoming_queue_|.
+    int enqueue_order_;
+  };
+
   // TaskQueue implementation.
   bool RunsTasksOnCurrentThread() const override;
   bool PostDelayedTask(const tracked_objects::Location& from_here,
@@ -50,12 +83,12 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
 
   void UpdateWorkQueue(LazyNow* lazy_now,
                        bool should_trigger_wakeup,
-                       const base::PendingTask* previous_task);
-  base::PendingTask TakeTaskFromWorkQueue();
+                       const Task* previous_task);
+  Task TakeTaskFromWorkQueue();
 
   void WillDeleteTaskQueueManager();
 
-  base::TaskQueue& work_queue() { return work_queue_; }
+  std::queue<Task>& work_queue() { return work_queue_; }
 
   WakeupPolicy wakeup_policy() const {
     DCHECK(main_thread_checker_.CalledOnValidThread());
@@ -70,16 +103,16 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
 
   void set_task_queue_set_index(size_t set_index) { set_index_ = set_index; }
 
-  // If the work queue isn't empty, |age| gets set to the sequence number of the
-  // front task and the dunctio returns true.  Otherwise the function returns
-  // false.
-  bool GetWorkQueueFrontTaskAge(int* age) const;
+  // If the work queue isn't empty, |enqueue_order| gets set to the enqueue
+  // order of the front task and the function returns true.  Otherwise the
+  // function returns false.
+  bool GetWorkQueueFrontTaskEnqueueOrder(int* enqueue_order) const;
 
   bool GetQuiescenceMonitored() const { return should_monitor_quiescence_; }
   bool GetShouldNotifyObservers() const { return should_notify_observers_; }
 
   // Test support functions.  These should not be used in production code.
-  void PushTaskOntoWorkQueueForTest(const base::PendingTask& task);
+  void PushTaskOntoWorkQueueForTest(const Task& task);
   void PopTaskFromWorkQueueForTest();
   size_t WorkQueueSizeForTest() const { return work_queue_.size(); }
 
@@ -126,20 +159,23 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   void ScheduleDelayedWorkLocked(LazyNow* lazy_now);
 
   void PumpQueueLocked();
-  bool TaskIsOlderThanQueuedTasks(const base::PendingTask* task);
+  bool TaskIsOlderThanQueuedTasks(const Task* task);
   bool ShouldAutoPumpQueueLocked(bool should_trigger_wakeup,
-                                 const base::PendingTask* previous_task);
+                                 const Task* previous_task);
+
+  enum class EnqueueOrderPolicy { SET_ENQUEUE_ORDER, DONT_SET_ENQUEUE_ORDER };
 
-  // Push the task onto the |incoming_queue_| and allocate a sequence number
-  // for it.
-  void EnqueueTaskLocked(const base::PendingTask& pending_task);
+  // Push the task onto the |incoming_queue_| and maybe allocate an
+  // enqueue_order for it based on |enqueue_order_policy|.
+  void EnqueueTaskLocked(const Task& pending_task,
+                         EnqueueOrderPolicy enqueue_order_policy);
 
   void TraceQueueSize(bool is_locked) const;
-  static void QueueAsValueInto(const base::TaskQueue& queue,
+  static void QueueAsValueInto(const std::queue<Task>& queue,
                                base::trace_event::TracedValue* state);
-  static void QueueAsValueInto(const base::DelayedTaskQueue& queue,
+  static void QueueAsValueInto(const std::priority_queue<Task>& queue,
                                base::trace_event::TracedValue* state);
-  static void TaskAsValueInto(const base::PendingTask& task,
+  static void TaskAsValueInto(const Task& task,
                               base::trace_event::TracedValue* state);
 
   // This lock protects all members in the contigious block below.
@@ -147,13 +183,9 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   mutable base::Lock lock_;
   base::PlatformThreadId thread_id_;
   TaskQueueManager* task_queue_manager_;
-  base::TaskQueue incoming_queue_;
+  std::queue<Task> incoming_queue_;
   PumpPolicy pump_policy_;
-  // Queue-local task sequence number for maintaining the order of delayed
-  // tasks which are posted for the exact same time. Note that this will be
-  // replaced by the global sequence number when the delay has elapsed.
-  int delayed_task_sequence_number_;
-  base::DelayedTaskQueue delayed_task_queue_;
+  std::priority_queue<Task> delayed_task_queue_;
   std::set<base::TimeTicks> in_flight_kick_delayed_tasks_;
 
   const char* name_;
@@ -161,7 +193,7 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   const char* disabled_by_default_verbose_tracing_category_;
 
   base::ThreadChecker main_thread_checker_;
-  base::TaskQueue work_queue_;
+  std::queue<Task> work_queue_;
   WakeupPolicy wakeup_policy_;
   size_t set_index_;
   bool should_monitor_quiescence_;