base: Cleanup class/struct forward declarations

base/synchronization/waitable_event_watcher.h

 // Copyright (c) 2012 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_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 #define BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_
 
 #include "base/base_export.h"
 #include "base/macros.h"
 #include "base/sequence_checker.h"
 #include "build/build_config.h"
 
 #if defined(OS_WIN)
 #include "base/win/object_watcher.h"
 #else
 #include "base/callback.h"
 #include "base/synchronization/waitable_event.h"
 #endif
 
 namespace base {
 
 class Flag;
 class AsyncWaiter;
-class AsyncCallbackTask;
 class WaitableEvent;
 
 // This class provides a way to wait on a WaitableEvent asynchronously.
 //
 // Each instance of this object can be waiting on a single WaitableEvent. When
 // the waitable event is signaled, a callback is invoked on the sequence that
 // called StartWatching(). This callback can be deleted by deleting the waiter.
 //
 // Typical usage:
 //
@@ skipping 74 matching lines @@
   // sequence.
   SequenceChecker sequence_checker_;
 #endif
 
   DISALLOW_COPY_AND_ASSIGN(WaitableEventWatcher);
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_WAITABLE_EVENT_WATCHER_H_