Add TaskScheduler::JoinForTesting().

base/threading/sequenced_worker_pool_unittest.cc

Index: base/threading/sequenced_worker_pool_unittest.cc
diff --git a/base/threading/sequenced_worker_pool_unittest.cc b/base/threading/sequenced_worker_pool_unittest.cc
index 6e203390c073e3a2e78b9e55bd9fd9986b9c4bf7..5f9c9bfd2ad86229083895c82a99d3c4df686934 100644
--- a/base/threading/sequenced_worker_pool_unittest.cc
+++ b/base/threading/sequenced_worker_pool_unittest.cc
@@ -19,7 +19,6 @@
 #include "base/synchronization/lock.h"
 #include "base/task_scheduler/scheduler_worker_pool_params.h"
 #include "base/task_scheduler/task_scheduler.h"
-#include "base/task_scheduler/task_scheduler_impl.h"
 #include "base/test/sequenced_task_runner_test_template.h"
 #include "base/test/sequenced_worker_pool_owner.h"
 #include "base/test/task_runner_test_template.h"
@@ -296,8 +295,7 @@ class SequencedWorkerPoolTest
   // Destroys and unregisters the registered TaskScheduler, if any.
   void DeleteTaskScheduler() {
     if (TaskScheduler::GetInstance()) {
-      static_cast<internal::TaskSchedulerImpl*>(TaskScheduler::GetInstance())
-          ->JoinForTesting();
+      TaskScheduler::GetInstance()->JoinForTesting();
       TaskScheduler::SetInstance(nullptr);
     }
   }