Reviewed by Gustavo Noronha Silva.

        [GTK] Bots are showing lots of GTK_IS_CONTAINER critical warnings
        https://bugs.webkit.org/show_bug.cgi?id=40990

        Instead of relying on the ScrollView's adjustment members to determine if a
        Scrollbar should be native or just a shell of the parent's scrollbar (main frame
        scrollbar), just check if this ScrollView has a parent. This will ensure the
        correct behavior when main frame scrollbar's are created after the ScrollView's
        containing adjustments go away.

        Lack of warnings during tests are the test for this fix.

        * platform/gtk/ScrollViewGtk.cpp:
        (WebCore::ScrollView::createScrollbar): Decide what type of scrollbar to make based
        on the result of the parent() method.
        * platform/gtk/ScrollbarGtk.cpp:
        (ScrollbarGtk::ScrollbarGtk): Allow for an m_adjustment which is null.
        (ScrollbarGtk::attachAdjustment): Ditto.
        (ScrollbarGtk::updateThumbPosition): Ditto.
        (ScrollbarGtk::updateThumbProportion): Ditto.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@65635 268f45cc-cd09-0410-ab3c-d52691b4dbfc