Add TaskScheduler::JoinForTesting().

base/task_scheduler/scheduler_worker_pool_impl_unittest.cc

Index: base/task_scheduler/scheduler_worker_pool_impl_unittest.cc
diff --git a/base/task_scheduler/scheduler_worker_pool_impl_unittest.cc b/base/task_scheduler/scheduler_worker_pool_impl_unittest.cc
index dfaa9c7ff3d3a8071aceda80584d98eb447622d9..5d6a594ace33956a3b3b31518d717b5aef2f498d 100644
--- a/base/task_scheduler/scheduler_worker_pool_impl_unittest.cc
+++ b/base/task_scheduler/scheduler_worker_pool_impl_unittest.cc
@@ -485,8 +485,6 @@ TEST_F(TaskSchedulerWorkerPoolSingleThreadedTest, SingleThreadTask) {
   // Give the worker pool a chance to reclaim its threads.
   PlatformThread::Sleep(kReclaimTimeForDetachTests + kExtraTimeToWaitForDetach);
 
-  worker_pool_->DisallowWorkerDetachmentForTesting();
-
   single_thread_task_runner->PostTask(
       FROM_HERE,
       Bind(&TaskSchedulerWorkerPoolSingleThreadedTest::CheckValidThread,
@@ -563,8 +561,6 @@ TEST_F(TaskSchedulerWorkerPoolCheckTlsReuse, CheckDetachedThreads) {
   // Give the worker pool a chance to detach its threads.
   PlatformThread::Sleep(kReclaimTimeForDetachTests + kExtraTimeToWaitForDetach);
 
-  worker_pool_->DisallowWorkerDetachmentForTesting();
-
   // Saturate and count the threads that do not have the magic TLS value. If the
   // value is not there, that means we're at a new thread.
   std::vector<std::unique_ptr<WaitableEvent>> count_waiters;
@@ -715,8 +711,10 @@ TEST_F(TaskSchedulerWorkerPoolHistogramTest, NumTasksBetweenWaitsWithDetach) {
   EXPECT_EQ(0, histogram->SnapshotSamples()->GetCount(10));
 
   tasks_can_exit_event.Signal();
+
+  // Do not let |tasks_can_exit_event| go out of scope before all workers are
+  // idle.
   worker_pool_->WaitForAllWorkersIdleForTesting();
-  worker_pool_->DisallowWorkerDetachmentForTesting();
 }
 
 namespace {
@@ -764,7 +762,6 @@ TEST_F(TaskSchedulerWorkerPoolHistogramTest, NumTasksBeforeDetach) {
 
   // Allow the SchedulerWorker that was associated with |task_runner| to detach.
   PlatformThread::Sleep(kReclaimTimeForDetachTests + kExtraTimeToWaitForDetach);
-  worker_pool_->DisallowWorkerDetachmentForTesting();
 
   // Verify that counts were recorded to the histogram as expected.
   const auto* histogram = worker_pool_->num_tasks_before_detach_histogram();