TaskScheduler: Add FlushForTesting().

base/task_scheduler/task_scheduler.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_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 41 matching lines @@
   // posted with the BLOCK_SHUTDOWN behavior will be run. When this returns:
   // - All SKIP_ON_SHUTDOWN tasks that were already running have completed their
   //   execution.
   // - All posted BLOCK_SHUTDOWN tasks have completed their execution.
   // - CONTINUE_ON_SHUTDOWN tasks might still be running.
   // Note that an implementation can keep threads and other resources alive to
   // support running CONTINUE_ON_SHUTDOWN after this returns. This can only be
   // called once.
   virtual void Shutdown() = 0;
 
+  // Waits until there are no pending undelayed tasks. May be called in tests
+  // to validate that a condition is met after all undelayed tasks have run.
+  //
+  // Does not wait for delayed tasks. Waits for undelayed tasks posted from
+  // other threads during the call. Returns immediately when shutdown completes.
+  virtual void FlushForTesting() = 0;
+
   // 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 deleted when a new
   // TaskScheduler is registered and is leaked on shutdown. The methods must
   // not be called when TaskRunners created by the previous TaskScheduler are
   // still alive. The methods are not thread-safe; proper synchronization is
   // required to use the post_task.h API after registering a new TaskScheduler.
 
   // Creates and sets a default task scheduler. CHECKs on failure.
   // |worker_pool_params_vector| describes the worker pools to create.
@@ skipping 10 matching lines @@
 
   // Retrieve the TaskScheduler set via CreateAndSetDefaultTaskScheduler() or
   // SetInstance(). This should be used very rarely; most users of TaskScheduler
   // should use the post_task.h API.
   static TaskScheduler* GetInstance();
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_