TaskScheduler: Add FlushForTesting().

base/task_scheduler/task_scheduler_impl.h

Index: base/task_scheduler/task_scheduler_impl.h
diff --git a/base/task_scheduler/task_scheduler_impl.h b/base/task_scheduler/task_scheduler_impl.h
index 2ab2f2962fd267a830b0f4a83d1abdf214f219d4..f49cccc80e4856833aab0a632b87971c14e9bcc8 100644
--- a/base/task_scheduler/task_scheduler_impl.h
+++ b/base/task_scheduler/task_scheduler_impl.h
@@ -59,6 +59,7 @@ class BASE_EXPORT TaskSchedulerImpl : public TaskScheduler {
       const TaskTraits& traits,
       ExecutionMode execution_mode) override;
   void Shutdown() override;
+  void FlushForTesting() override;
 
   // Joins all threads of this scheduler. Tasks that are already running are
   // allowed to complete their execution. This can only be called once.