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..3e6aea4e521218393c63af049130d09707627691 100644
--- a/components/scheduler/child/task_queue_impl.h
+++ b/components/scheduler/child/task_queue_impl.h
@@ -27,6 +27,18 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
                 const char* disabled_by_default_tracing_category,
                 const char* disabled_by_default_verbose_tracing_category);
 
+  struct Task : public base::PendingTask {
+    Task(const tracked_objects::Location& posted_from,
+         const base::Closure& task,
+         int sequence_number,
+         bool nestable);
+
+    // 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,
@@ -51,11 +63,11 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   void UpdateWorkQueue(LazyNow* lazy_now,
                        bool should_trigger_wakeup,
                        const base::PendingTask* previous_task);
-  base::PendingTask TakeTaskFromWorkQueue();
+  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 +82,16 @@ 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 sequence

[Sami, 2015/08/05 14:51:15]

nit: s/sequence number/enqueue order/ right?

[alex clarke (OOO till 29th), 2015/08/05 15:33:05]

Done.

+  // number 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(); }
 
@@ -130,16 +142,19 @@ class SCHEDULER_EXPORT TaskQueueImpl final : public TaskQueue {
   bool ShouldAutoPumpQueueLocked(bool should_trigger_wakeup,
                                  const base::PendingTask* previous_task);
 
-  // Push the task onto the |incoming_queue_| and allocate a sequence number
-  // for it.
-  void EnqueueTaskLocked(const base::PendingTask& pending_task);
+  enum class EnqueueOrderPolicy { SET_ENQUEUE_ORDER, DONT_SET_ENQUEUE_ORDER };
+
+  // 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 +162,9 @@ 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 +172,7 @@ 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_;