Reviewed by Eric Seidel.  Landed by Adam Barth.

        https://bugs.webkit.org/show_bug.cgi?id=26333
        Alert during a dragenter event handler will crash the renderer
        
        This crash is casued by calling NULL pointer m_documentUnderMouse in
        DragController::tryDocumentDrag()

        tryDHTMLDrag fires dragenter event. The event listener that listens
        to this event may create a nested message loop (open a modal dialog),
        which could process dragleave event and reset m_documentUnderMouse in
        dragExited.

        Fix the crash by checking m_documentUnderMouse after tryDHTMLDrag and
        do not continue if the pointer has been set to NULL.

        Test: DRT does not show alerts so add a manual test:
              manual-tests/drag-enter-alert.html

        * manual-tests/drag-enter-alert.html: Added.
        * manual-tests/resources/drag-image.png: Added.
        * page/DragController.cpp:
        (WebCore::DragController::tryDocumentDrag):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44659 268f45cc-cd09-0410-ab3c-d52691b4dbfc