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

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index ad780003e195e36cd9662f15e938a90dfee4326a..61e760519fa599d582fb074cc31e4a365c36d99c 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -783,8 +783,6 @@ component("base") {
     "task_runner.cc",
     "task_runner.h",
     "task_runner_util.h",
-    "task_scheduler/delayed_task_manager.cc",
-    "task_scheduler/delayed_task_manager.h",
     "task_scheduler/post_task.cc",
     "task_scheduler/post_task.h",
     "task_scheduler/priority_queue.cc",
@@ -792,8 +790,6 @@ component("base") {
     "task_scheduler/scheduler_lock.h",
     "task_scheduler/scheduler_lock_impl.cc",
     "task_scheduler/scheduler_lock_impl.h",
-    "task_scheduler/scheduler_service_thread.cc",
-    "task_scheduler/scheduler_service_thread.h",
     "task_scheduler/scheduler_worker.cc",
     "task_scheduler/scheduler_worker.h",
     "task_scheduler/scheduler_worker_pool.h",
@@ -1930,10 +1926,8 @@ test("base_unittests") {
     "system_monitor/system_monitor_unittest.cc",
     "task/cancelable_task_tracker_unittest.cc",
     "task_runner_util_unittest.cc",
-    "task_scheduler/delayed_task_manager_unittest.cc",
     "task_scheduler/priority_queue_unittest.cc",
     "task_scheduler/scheduler_lock_unittest.cc",
-    "task_scheduler/scheduler_service_thread_unittest.cc",
     "task_scheduler/scheduler_worker_pool_impl_unittest.cc",
     "task_scheduler/scheduler_worker_stack_unittest.cc",
     "task_scheduler/scheduler_worker_unittest.cc",