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 385edd77a8bbf4df73a043753c624d19d1c0afb1..febb49a52101faf317876c8ac8a2a44f24c8498c 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;
+
   // 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 deleted when a new