content: Cleanup class/struct forward declarations

content/child/fileapi/webfilesystem_impl.h

 // Copyright 2013 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 CONTENT_CHILD_FILEAPI_WEBFILESYSTEM_IMPL_H_
 #define CONTENT_CHILD_FILEAPI_WEBFILESYSTEM_IMPL_H_
 
 #include <map>
 
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/threading/non_thread_safe.h"
 #include "content/public/child/worker_thread.h"
 #include "third_party/WebKit/public/platform/WebFileSystem.h"
 
 namespace base {
-class WaitableEvent;
 class SingleThreadTaskRunner;
 }
 
 namespace blink {
 class WebURL;
 class WebFileWriter;
 class WebFileWriterClient;
 }
 
 namespace content {
@@ skipping 82 matching lines @@
   CallbacksMap callbacks_;
   int next_callbacks_id_;
   WaitableCallbackResultsMap waitable_results_;
 
   DISALLOW_COPY_AND_ASSIGN(WebFileSystemImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_CHILD_FILEAPI_WEBFILESYSTEM_IMPL_H_