Run ScopedTaskScheduler tasks synchronously.

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 496c567b13ee0cb9e5933461360a3f0fad60a4ef..6e203390c073e3a2e78b9e55bd9fd9986b9c4bf7 100644
--- a/base/threading/sequenced_worker_pool_unittest.cc
+++ b/base/threading/sequenced_worker_pool_unittest.cc
@@ -296,7 +296,8 @@ class SequencedWorkerPoolTest
   // Destroys and unregisters the registered TaskScheduler, if any.
   void DeleteTaskScheduler() {
     if (TaskScheduler::GetInstance()) {
-      TaskScheduler::GetInstance()->JoinForTesting();
+      static_cast<internal::TaskSchedulerImpl*>(TaskScheduler::GetInstance())
+          ->JoinForTesting();
       TaskScheduler::SetInstance(nullptr);
     }
   }