TaskScheduler: Add FlushForTesting().

base/task_scheduler/task_scheduler_impl.cc

Index: base/task_scheduler/task_scheduler_impl.cc
diff --git a/base/task_scheduler/task_scheduler_impl.cc b/base/task_scheduler/task_scheduler_impl.cc
index d93c62023e8107b963f6747501dd253fd0da1173..70a71baf7ce45d9ab1ce3632071b863f9cc40b11 100644
--- a/base/task_scheduler/task_scheduler_impl.cc
+++ b/base/task_scheduler/task_scheduler_impl.cc
@@ -57,6 +57,10 @@ void TaskSchedulerImpl::Shutdown() {
   task_tracker_.Shutdown();
 }
 
+void TaskSchedulerImpl::FlushForTesting() {
+  task_tracker_.Flush();
+}
+
 void TaskSchedulerImpl::JoinForTesting() {
 #if DCHECK_IS_ON()
   DCHECK(!join_for_testing_returned_.IsSet());