Introduce SchedulerSingleThreadTaskRunnerManager

base/task_scheduler/scheduler_worker.cc

Index: base/task_scheduler/scheduler_worker.cc
diff --git a/base/task_scheduler/scheduler_worker.cc b/base/task_scheduler/scheduler_worker.cc
index 3e77436b984aaf9d5ac2653979ed67d1770cf8de..3f371887d511454e648054c7bf98d3255ee3ce20 100644
--- a/base/task_scheduler/scheduler_worker.cc
+++ b/base/task_scheduler/scheduler_worker.cc
@@ -67,7 +67,6 @@ class SchedulerWorker::Thread : public PlatformThread::Delegate {
         if (outer_->delegate_->CanDetach(outer_.get())) {
           detached_thread = outer_->DetachThreadObject(DetachNotify::DELEGATE);
           if (detached_thread) {
-            outer_ = nullptr;
             DCHECK_EQ(detached_thread.get(), this);
             PlatformThread::Detach(thread_handle_);
             break;
@@ -119,6 +118,8 @@ class SchedulerWorker::Thread : public PlatformThread::Delegate {
     //           nullptr. JoinForTesting() cleans up if we get nullptr.
     if (!detached_thread)
       detached_thread = outer_->DetachThreadObject(DetachNotify::SILENT);
+
+    outer_->delegate_->OnMainExit();
   }
 
   void Join() { PlatformThread::Join(thread_handle_); }