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

components/scheduler/child/task_queue_manager_unittest.cc

Index: components/scheduler/child/task_queue_manager_unittest.cc
diff --git a/components/scheduler/child/task_queue_manager_unittest.cc b/components/scheduler/child/task_queue_manager_unittest.cc
index d9eb15416ca36ee2dac0f1c65dfbaaf036046099..0bd2b7517ee0b64816fb34a262d83679b0b7cd23 100644
--- a/components/scheduler/child/task_queue_manager_unittest.cc
+++ b/components/scheduler/child/task_queue_manager_unittest.cc
@@ -1029,4 +1029,46 @@ TEST_F(TaskQueueManagerTest, QuitWhileNested) {
   EXPECT_FALSE(was_nested);
 }
 
+class SequenceNumberCapturingTaskObserver
+    : public base::MessageLoop::TaskObserver {
+ public:
+  // MessageLoop::TaskObserver overrides.
+  void WillProcessTask(const base::PendingTask& pending_task) override {}
+  void DidProcessTask(const base::PendingTask& pending_task) override {
+    sequence_numbers_.push_back(pending_task.sequence_num);
+  }
+
+  const std::vector<int>& sequence_numbers() const { return sequence_numbers_; }
+
+ private:
+  std::vector<int> sequence_numbers_;
+};
+
+TEST_F(TaskQueueManagerTest, SequenceNumSetWhenTaskIsPosted) {
+  Initialize(1u);
+
+  SequenceNumberCapturingTaskObserver observer;
+  manager_->AddTaskObserver(&observer);
+
+  // Register four tasks that will run in reverse order.
+  std::vector<int> run_order;
+  runners_[0]->PostDelayedTask(FROM_HERE, base::Bind(&TestTask, 1, &run_order),
+                               base::TimeDelta::FromMilliseconds(30));
+  runners_[0]->PostDelayedTask(FROM_HERE, base::Bind(&TestTask, 2, &run_order),
+                               base::TimeDelta::FromMilliseconds(20));
+  runners_[0]->PostDelayedTask(FROM_HERE, base::Bind(&TestTask, 3, &run_order),
+                               base::TimeDelta::FromMilliseconds(10));
+  runners_[0]->PostTask(FROM_HERE, base::Bind(&TestTask, 4, &run_order));
+
+  test_task_runner_->RunForPeriod(base::TimeDelta::FromMilliseconds(40));
+  ASSERT_THAT(run_order, ElementsAre(4, 3, 2, 1));
+
+  // The sequence numbers are a zero-based monotonically incrememting counter
+  // which should be set when the task is posted rather than when it's enqueued
+  // onto the incomming queue.
+  EXPECT_THAT(observer.sequence_numbers(), ElementsAre(3, 2, 1, 0));
+
+  manager_->RemoveTaskObserver(&observer);
+}
+
 }  // namespace scheduler