Prevent leak on completion of drop operation.

Source/core/page/DragController.cpp

 /*
  * Copyright (C) 2007, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Google Inc.
  *
  * 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
@@ skipping 233 matching lines @@
         bool preventedDefault = false;
         if (mainFrame->view()) {
             // Sending an event can result in the destruction of the view and part.
             RefPtrWillBeRawPtr<DataTransfer> dataTransfer = createDraggingDataTransfer(DataTransferReadable, dragData);
             dataTransfer->setSourceOperation(dragData->draggingSourceOperationMask());
             preventedDefault = mainFrame->eventHandler().performDragAndDrop(createMouseEvent(dragData), dataTransfer.get());
             dataTransfer->setAccessPolicy(DataTransferNumb); // Invalidate clipboard here for security
         }
         if (preventedDefault) {
             m_documentUnderMouse = nullptr;
+            cancelDrag();
             return true;
         }
     }
 
     if ((m_dragDestinationAction & DragDestinationActionEdit) && concludeEditDrag(dragData)) {
         m_documentUnderMouse = nullptr;
         return true;
     }
 
     m_documentUnderMouse = nullptr;
@@ skipping 721 matching lines @@
 
 void DragController::trace(Visitor* visitor)
 {
     visitor->trace(m_page);
     visitor->trace(m_documentUnderMouse);
     visitor->trace(m_dragInitiator);
     visitor->trace(m_fileInputElementUnderMouse);
 }
 
 } // namespace blink