base: Cleanup class/struct forward declarations

base/task_scheduler/test_task_factory.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_TEST_TASK_FACTORY_H_
 #define BASE_TASK_SCHEDULER_TEST_TASK_FACTORY_H_
 
 #include <stddef.h>
 
 #include <unordered_set>
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/synchronization/condition_variable.h"
 #include "base/synchronization/lock.h"
 #include "base/task_runner.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/task_scheduler/test_utils.h"
 #include "base/threading/thread_checker_impl.h"
 
 namespace base {
-
-class WaitableEvent;
-
 namespace internal {
 namespace test {
 
 // A TestTaskFactory posts tasks to a TaskRunner and verifies that they run as
 // expected. Generates a test failure when:
 // - The RunsTasksOnCurrentThread() method of the TaskRunner returns false on a
 //   thread on which a Task is run.
 // - The TaskRunnerHandles set in the context of the task don't match what's
 //   expected for the tested ExecutionMode.
 // - The ExecutionMode of the TaskRunner is SEQUENCED or SINGLE_THREADED and
@@ skipping 57 matching lines @@
   ThreadCheckerImpl thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(TestTaskFactory);
 };
 
 }  // namespace test
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TEST_TASK_FACTORY_H_