Reviewed by Adam Barth.

        Crash in DOMWindow::clearTimeout etc when DOMWindow is not connected to a frame.
        https://bugs.webkit.org/show_bug.cgi?id=29832

        Need to make sure the script caches are reset when frame gets disconnected from still-alive DOMWindow.
        This will prevent JS from calling DOMWindow methods that can not be completed w/o the frame.

        I am not sure it's possible to test this since the only file that repros the problem need ~10 seconds to cause crash.

        * page/Frame.cpp:
        (WebCore::Frame::~Frame): Right after frame disconnects from DOMWindow, clear WindowShell.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@49786 268f45cc-cd09-0410-ab3c-d52691b4dbfc