TaskScheduler: Replace the SchedulerServiceThread with a base::Thread.

base/task_scheduler/scheduler_worker_pool_impl.cc

Index: base/task_scheduler/scheduler_worker_pool_impl.cc
diff --git a/base/task_scheduler/scheduler_worker_pool_impl.cc b/base/task_scheduler/scheduler_worker_pool_impl.cc
index f14e5f9249a550e102cfa72ef2944ad749fcf113..4a9ee43fb08fec7f4a78868cda81336c5e37e5ff 100644
--- a/base/task_scheduler/scheduler_worker_pool_impl.cc
+++ b/base/task_scheduler/scheduler_worker_pool_impl.cc
@@ -416,7 +416,7 @@ void SchedulerWorkerPoolImpl::PostTaskWithSequenceNow(
 
   // Confirm that |task| is ready to run (its delayed run time is either null or
   // in the past).
-  DCHECK_LE(task->delayed_run_time, delayed_task_manager_->Now());
+  DCHECK_LE(task->delayed_run_time, TimeTicks::Now());
 
   // Because |worker| belongs to this worker pool, we know that the type
   // of its delegate is SchedulerWorkerDelegateImpl.