Fix a document memory leak in drag-and-drop.

Source/core/clipboard/Clipboard.cpp

 /*
  * Copyright (C) 2006, 2007, 2008 Apple 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:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 210 matching lines @@
         exceptionState.throwTypeError("setDragImage: Invalid first argument");
         return;
     }
     IntPoint location(x, y);
     if (isHTMLImageElement(*image) && !image->inDocument())
         setDragImageResource(toHTMLImageElement(*image).cachedImage(), location);
     else
         setDragImageElement(image, location);
 }
 
+void Clipboard::clearDragImage()
+{
+    if (!canSetDragImage())
+        return;
+
+    m_dragImage = 0;
+    m_dragLoc = IntPoint();
+    m_dragImageElement = nullptr;
+}
+
 void Clipboard::setDragImageResource(ImageResource* img, const IntPoint& loc)
 {
     setDragImage(img, 0, loc);
 }
 
 void Clipboard::setDragImageElement(Node* node, const IntPoint& loc)
 {
     setDragImage(0, node, loc);
 }
 
@@ skipping 282 matching lines @@
         return String("copy");
     }
 }
 
 void Clipboard::trace(Visitor* visitor)
 {
     visitor->trace(m_dataObject);
 }
 
 } // namespace WebCore