Refactor WebCore::DragImage.

Source/core/platform/chromium/DragImageRef.h

-// Copyright (c) 2008, 2009, Google Inc. All rights reserved.
-// 
-// Redistribution and use in source and binary forms, with or without
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-// met:
-// 
-//     * Redistributions of source code must retain the above copyright
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-
-#ifndef DragImageRef_h
-#define DragImageRef_h
-
-class SkBitmap;
-
-namespace WebCore {
-
-struct DragImageChromium {
-    SkBitmap* bitmap;
-    float resolutionScale;
-};
-
-typedef DragImageChromium* DragImageRef;
-
-} // namespace WebCore
-
-#endif