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

components/scheduler/child/task_queue_manager_unittest.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 "base/location.h"
 #include "base/run_loop.h"
 #include "base/single_thread_task_runner.h"
 #include "base/test/simple_test_tick_clock.h"
@@ skipping 1011 matching lines @@
   base::RunLoop run_loop;
   runners_[0]->PostTask(
       FROM_HERE,
       base::Bind(&PostAndQuitFromNestedRunloop, base::Unretained(&run_loop),
                  runners_[0], base::Unretained(&was_nested)));
 
   message_loop_->RunUntilIdle();
   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