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

Source/core/page/EventHandler.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Alexey Proskuryakov (ap@webkit.org)
  * Copyright (C) 2012 Digia Plc. and/or its subsidiary(-ies)
  *
  * 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.
@@ skipping 3019 matching lines @@
         break;
     case DragSourceActionNone:
         ASSERT_NOT_REACHED();
     }
 
     return abs(delta.width()) >= threshold || abs(delta.height()) >= threshold;
 }
 
 void EventHandler::freeClipboard()
 {
-    if (dragState().m_dragClipboard)
+    if (dragState().m_dragClipboard) {
+        dragState().m_dragClipboard->clearDragImage();
         dragState().m_dragClipboard->setAccessPolicy(ClipboardNumb);
+    }
 }
 
 void EventHandler::dragSourceEndedAt(const PlatformMouseEvent& event, DragOperation operation)
 {
     // Send a hit test request so that RenderLayer gets a chance to update the :hover and :active pseudoclasses.
     HitTestRequest request(HitTestRequest::Release | HitTestRequest::ConfusingAndOftenMisusedDisallowShadowContent);
     prepareMouseEvent(request, event);
 
     if (dragState().m_dragSrc) {
         dragState().m_dragClipboard->setDestinationOperation(operation);
@@ skipping 679 matching lines @@
 unsigned EventHandler::accessKeyModifiers()
 {
 #if OS(MACOSX)
     return PlatformEvent::CtrlKey | PlatformEvent::AltKey;
 #else
     return PlatformEvent::AltKey;
 #endif
 }
 
 } // namespace WebCore