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

base/task_scheduler/task.h

Index: base/task_scheduler/task.h
diff --git a/base/task_scheduler/task.h b/base/task_scheduler/task.h
index b2fa7661b569686767e013af7c6c1d23559d283e..c014671ce076cfe9e30665f4d6ccb885e4511758 100644
--- a/base/task_scheduler/task.h
+++ b/base/task_scheduler/task.h
@@ -36,6 +36,9 @@ struct BASE_EXPORT Task : public PendingTask {
   // The TaskTraits of this task.
   const TaskTraits traits;
 
+  // The delay that must expire before the task runs.
+  const TimeDelta delay;
+
   // The time at which the task was inserted in its sequence. For an undelayed
   // task, this happens at post time. For a delayed task, this happens some
   // time after the task's delay has expired. If the task hasn't been inserted