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 c484f3e0b5fb8999f618523d209fb2d788f94f32..29475b94bc05642595f641eae33b721d1dfcf973 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -83,6 +83,10 @@ 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