Reviewed by Adele.

        Fixes DragController to allow a null target Document.
        This is encountered in new tabs/windows that have not
        loaded any page (including about:blank)

        * page/DragController.cpp:
        (WebCore::documentAtPoint):
        (WebCore::DragController::tryDocumentDrag):
        (WebCore::DragController::operationForLoad):
        (WebCore::DragController::concludeDrag):
        * page/mac/DragControllerMac.mm:
        (WebCore::DragController::dragOperation):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@19084 268f45cc-cd09-0410-ab3c-d52691b4dbfc