Create base::test::ScopedTaskScheduler.

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index d4084c6f8a86af0d25a060c76fd958588e341aa7..cbfaae900dd05ecab658997a0176a232a5a261d0 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -82,6 +82,10 @@ class BASE_EXPORT TaskScheduler {
   // other threads during the call. Returns immediately when shutdown completes.
   virtual void FlushForTesting() = 0;
 
+  // Joins all threads of this scheduler. Tasks that are already running are
+  // allowed to complete their execution. This can only be called once.
+  virtual void JoinForTesting() = 0;
+
   // CreateAndSetSimpleTaskScheduler(), CreateAndSetDefaultTaskScheduler(), and
   // SetInstance() register a TaskScheduler to handle tasks posted through the
   // post_task.h API for this process. The registered TaskScheduler will only be