When SeaMonkey switched to toolkit's nsDragAndDrop.js, it lost the dragDropSecurityCheck method, which for SM was on nsDragAndDrop, but for FF was on tabbrowser.
Moving that method from tabbrowser to toolkit's nsDragAndDrop.js, and cleaning it up a little.
git-svn-id: svn://10.0.0.236/trunk@231034 18797224-902f-48f8-a5cc-f745e15eee43