Fix: https://bugs.webkit.org/show_bug.cgi?id=26723
        Where the m_mouseDown event was never being set on windows, so the
        client X and Y coordinates were always being reported as zero in a
        dragstart handler.
        
        Reviewed by Mark Rowe.

        Test: editing/selection/drag-start-event-client-x-y.html

        * page/EventHandler.cpp:
        (WebCore::EventHandler::handleMousePressEvent):
        Set the m_mouseDown event when the mouse press is handled.
        * page/mac/EventHandlerMac.mm:
        (WebCore::EventHandler::mouseDown):
        Removed now redundant setting of m_mouseDown.
        
        Layout test for making sure the client X Y coordinates of the event
        in a dragstart handler are correct.
        
        https://bugs.webkit.org/show_bug.cgi?id=26723

        * editing/selection/drag-start-event-client-x-y-expected.txt: Added.
        * editing/selection/drag-start-event-client-x-y.html: Added.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@45278 268f45cc-cd09-0410-ab3c-d52691b4dbfc