Reviewed by Andreas Kling.

        Setting value of m_PressedPos to make scrolling smooth

        Page scroll popup menu "Scroll here" option not working when cliking above scroll slider/handler.
        https://bugs.webkit.org/show_bug.cgi?id=51349

        The value of m_PressedPos was getting set before moveThumb() call
        in all other scenarios except when "Scroll Here" option is used.
        Hence scrolling with this option was not as expected even in cases
        where scrolling was happening. The thumb would move in unexpected
        direction. m_PressedPos is now set to pressed position so delta is
        calculated.
        Unable to write a test case as the test needs to click on "Scroll
        Here" option of context sensitive menu and QTest is unable to do it.
        Besides no new functionality introduced.

        * platform/qt/ScrollbarQt.cpp:
        (WebCore::Scrollbar::contextMenu):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76946 268f45cc-cd09-0410-ab3c-d52691b4dbfc