Introduce SchedulerSingleThreadTaskRunnerManager

base/BUILD.gn

Index: base/BUILD.gn
diff --git a/base/BUILD.gn b/base/BUILD.gn
index 21c303dcb10b50f3cf239d12aa30252b7d242765..8fb6be525953c969d476907e6abf20aaa2aa5fcb 100644
--- a/base/BUILD.gn
+++ b/base/BUILD.gn
@@ -850,6 +850,8 @@ component("base") {
     "task_scheduler/scheduler_lock.h",
     "task_scheduler/scheduler_lock_impl.cc",
     "task_scheduler/scheduler_lock_impl.h",
+    "task_scheduler/scheduler_single_thread_task_runner_manager.cc",
+    "task_scheduler/scheduler_single_thread_task_runner_manager.h",
     "task_scheduler/scheduler_worker.cc",
     "task_scheduler/scheduler_worker.h",
     "task_scheduler/scheduler_worker_pool.h",
@@ -2088,6 +2090,7 @@ test("base_unittests") {
     "task_scheduler/delayed_task_manager_unittest.cc",
     "task_scheduler/priority_queue_unittest.cc",
     "task_scheduler/scheduler_lock_unittest.cc",
+    "task_scheduler/scheduler_single_thread_task_runner_manager_unittest.cc",
     "task_scheduler/scheduler_worker_pool_impl_unittest.cc",
     "task_scheduler/scheduler_worker_stack_unittest.cc",
     "task_scheduler/scheduler_worker_unittest.cc",