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..53593953c08ce94d47a73f7471204dfbd941f349 100644
--- a/components/scheduler/child/task_queue_impl.h
+++ b/components/scheduler/child/task_queue_impl.h
@@ -20,6 +20,17 @@ class TaskQueueManager;
 
 namespace internal {
 
+struct SchedulerTask : public base::PendingTask {

[Sami, 2015/08/05 11:27:34]

Sorry to go all bikeshed on this but I think TQM is still distinct from the
scheduler and at a level below. How about just calling this PendingTask or
simply Task?

[alex clarke (OOO till 29th), 2015/08/05 14:32:43]

Done.

+  SchedulerTask();
+  SchedulerTask(const tracked_objects::Location& posted_from,
+                const base::Closure& task,
+                bool nestable);
+
+  // Similar to sequence number, but the task age is not defined for delayed
+  // tasks until enqueued on the |incomming_queue_|.

[Sami, 2015/08/05 11:27:34]

typo: incoming

[alex clarke (OOO till 29th), 2015/08/05 14:32:43]

Done.

+  int age;
+};
+
 class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
  public:
   TaskQueueImpl(TaskQueueManager* task_queue_manager,
@@ -50,12 +61,12 @@ 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 SchedulerTask* previous_task);
+  SchedulerTask TakeTaskFromWorkQueue();
 
   void WillDeleteTaskQueueManager();
 
-  base::TaskQueue& work_queue() { return work_queue_; }
+  std::queue<SchedulerTask>& work_queue() { return work_queue_; }
 
   WakeupPolicy wakeup_policy() const {
     DCHECK(main_thread_checker_.CalledOnValidThread());
@@ -79,7 +90,7 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   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 SchedulerTask& task);
   void PopTaskFromWorkQueueForTest();
   size_t WorkQueueSizeForTest() const { return work_queue_.size(); }
 
@@ -126,20 +137,20 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   void ScheduleDelayedWorkLocked(LazyNow* lazy_now);
 
   void PumpQueueLocked();
-  bool TaskIsOlderThanQueuedTasks(const base::PendingTask* task);
+  bool TaskIsOlderThanQueuedTasks(const SchedulerTask* task);
   bool ShouldAutoPumpQueueLocked(bool should_trigger_wakeup,
-                                 const base::PendingTask* previous_task);
+                                 const SchedulerTask* previous_task);
 
   // Push the task onto the |incoming_queue_| and allocate a sequence number
   // for it.
-  void EnqueueTaskLocked(const base::PendingTask& pending_task);
+  void EnqueueTaskLocked(const SchedulerTask& pending_task);
 
   void TraceQueueSize(bool is_locked) const;
-  static void QueueAsValueInto(const base::TaskQueue& queue,
+  static void QueueAsValueInto(const std::queue<SchedulerTask>& queue,
                                base::trace_event::TracedValue* state);
-  static void QueueAsValueInto(const base::DelayedTaskQueue& queue,
+  static void QueueAsValueInto(const std::priority_queue<SchedulerTask>& queue,
                                base::trace_event::TracedValue* state);
-  static void TaskAsValueInto(const base::PendingTask& task,
+  static void TaskAsValueInto(const SchedulerTask& task,
                               base::trace_event::TracedValue* state);
 
   // This lock protects all members in the contigious block below.
@@ -147,13 +158,9 @@ 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<SchedulerTask> 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<SchedulerTask> delayed_task_queue_;
   std::set<base::TimeTicks> in_flight_kick_delayed_tasks_;
 
   const char* name_;
@@ -161,7 +168,7 @@ 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<SchedulerTask> work_queue_;
   WakeupPolicy wakeup_policy_;
   size_t set_index_;
   bool should_monitor_quiescence_;