Run ScopedTaskScheduler tasks synchronously.

base/task_scheduler/task_scheduler_impl.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 44 matching lines @@
                           const Closure& task) override;
   scoped_refptr<TaskRunner> CreateTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   scoped_refptr<SequencedTaskRunner> CreateSequencedTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   scoped_refptr<SingleThreadTaskRunner> CreateSingleThreadTaskRunnerWithTraits(
       const TaskTraits& traits) override;
   std::vector<const HistogramBase*> GetHistograms() const override;
   void Shutdown() override;
   void FlushForTesting() override;
-  void JoinForTesting() override;
+
+  // Joins all threads. Tasks that are already running are allowed to complete
+  // their execution. This can only be called once.
+  void JoinForTesting();
 
  private:
   explicit TaskSchedulerImpl(const WorkerPoolIndexForTraitsCallback&
                                  worker_pool_index_for_traits_callback);
 
   void Initialize(
       const std::vector<SchedulerWorkerPoolParams>& worker_pool_params_vector);
 
   // Returns the worker pool that runs Tasks with |traits|.
   SchedulerWorkerPool* GetWorkerPoolForTraits(const TaskTraits& traits);
@@ skipping 13 matching lines @@
   AtomicFlag join_for_testing_returned_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(TaskSchedulerImpl);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_IMPL_H_