Reviewed by Oliver Hunt.

        Add a new platform specific method cleanupAfterSystemDrag
        that platforms can ue to clean up drag state after a
        system drag.  Use this method on PLAFORM(MAC).
        https://bugs.webkit.org/show_bug.cgi?id=21956

        * ChangeLog:
        * page/DragController.cpp:
        (WebCore::DragController::doSystemDrag):
        * page/DragController.h:
        * page/gtk/DragControllerGtk.cpp:
        (WebCore::DragController::cleanupAfterSystemDrag):
        * page/mac/DragControllerMac.mm:
        (WebCore::DragController::cleanupAfterSystemDrag):
        * page/qt/DragControllerQt.cpp:
        (WebCore::DragController::cleanupAfterSystemDrag):
        * page/win/DragControllerWin.cpp:
        (WebCore::DragController::cleanupAfterSystemDrag):
        * page/wx/DragControllerWx.cpp:
      (WebCore::DragController::cleanupAfterSystemDrag):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@38805 268f45cc-cd09-0410-ab3c-d52691b4dbfc