Clean up forward declarations in Source/core

Source/core/clipboard/DataTransferItem.h

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 24 matching lines @@
 #include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
 namespace blink {
 
 class Blob;
 class DataObjectItem;
 class DataTransfer;
-class File;
 class StringCallback;
 class ExecutionContext;
 
 class DataTransferItem final : public RefCountedWillBeGarbageCollected<DataTransferItem>, public ScriptWrappable {
     DECLARE_EMPTY_DESTRUCTOR_WILL_BE_REMOVED(DataTransferItem);
     DEFINE_WRAPPERTYPEINFO();
 public:
     static PassRefPtrWillBeRawPtr<DataTransferItem> create(PassRefPtrWillBeRawPtr<DataTransfer>, PassRefPtrWillBeRawPtr<DataObjectItem>);
 
     String kind() const;
@@ skipping 10 matching lines @@
 private:
     DataTransferItem(PassRefPtrWillBeRawPtr<DataTransfer>, PassRefPtrWillBeRawPtr<DataObjectItem>);
 
     RefPtrWillBeMember<DataTransfer> m_dataTransfer;
     RefPtrWillBeMember<DataObjectItem> m_item;
 };
 
 } // namespace blink
 
 #endif // DataTransferItem_h