Add TaskScheduler::JoinForTesting().

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index 5d9344bcbad7e52502f645ce3001026621d1dbe2..a73929cc88938f50a223e934188820b463c8db6d 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -38,6 +38,9 @@ class BASE_EXPORT TaskScheduler {
   using WorkerPoolIndexForTraitsCallback =
       Callback<size_t(const TaskTraits& traits)>;
 
+  // Destroying a TaskScheduler is not allowed in production; it is always
+  // leaked. In tests, it should only be destroyed after JoinForTesting() has
+  // returned.
   virtual ~TaskScheduler() = default;
 
   // Posts |task| with a |delay| and specific |traits|. |delay| can be zero.
@@ -85,6 +88,10 @@ class BASE_EXPORT TaskScheduler {
   // other threads during the call. Returns immediately when shutdown completes.
   virtual void FlushForTesting() = 0;
 
+  // Joins all threads. Tasks that are already running are allowed to complete
+  // their execution. This can only be called once.
+  virtual void JoinForTesting() = 0;
+
   // CreateAndSetSimpleTaskScheduler(), CreateAndSetDefaultTaskScheduler(), and
   // SetInstance() register a TaskScheduler to handle tasks posted through the
   // post_task.h API for this process. The registered TaskScheduler will only be