Separate the create and start phases in SchedulerSingleThreadTaskRunnerManager.

base/task_scheduler/task_scheduler_impl.cc

Index: base/task_scheduler/task_scheduler_impl.cc
diff --git a/base/task_scheduler/task_scheduler_impl.cc b/base/task_scheduler/task_scheduler_impl.cc
index 9d0320caf1a247026bc22f7753c78f67f56f55bc..6a2732d2e15cd60a61155c14f0dc5918bbefc369 100644
--- a/base/task_scheduler/task_scheduler_impl.cc
+++ b/base/task_scheduler/task_scheduler_impl.cc
@@ -240,6 +240,7 @@ void TaskSchedulerImpl::Initialize(
       MakeUnique<SchedulerSingleThreadTaskRunnerManager>(
           worker_pool_params_vector, worker_pool_index_for_traits_callback_,
           task_tracker_.get(), delayed_task_manager_.get());
+  single_thread_task_runner_manager_->Start();
 
   // Callback invoked by workers to re-enqueue a sequence in the appropriate
   // PriorityQueue.