[9] Review request for 8136999[macosx] NSException and NPE in a crash test

[Alexander Scherbatiy]

Hello,
Could you review the fix:
  bug: https://bugs.openjdk.java.net/browse/JDK-8136999
  webrev: http://cr.openjdk.java.net/~alexsch/8136999/webrev.00

  The test sets drop target to null in the drop handling which leads to 
the drop target resources disposing.
  The fix moves events flushing to the 
CDropTargetContextPeer.handleDropMessage() method.

Thanks,
Alexandr.