Reviewed by Sam Weinig.

        Implement NPN_PopUpContextMenu
        https://bugs.webkit.org/show_bug.cgi?id=52646
        <rdar://problem/8735616>

        * WebProcess/Plugins/Netscape/NetscapeBrowserFuncs.cpp:
        (WebKit::NPN_PopUpContextMenu):
        Call NetscapePlugin::popUpContextMenu.

        * WebProcess/Plugins/Netscape/NetscapePlugin.cpp:
        (WebKit::NetscapePlugin::NetscapePlugin):
        Initialize m_currentMouseEvent.

        * WebProcess/Plugins/Netscape/NetscapePlugin.h:
        Add m_currentMouseEvent.

        * WebProcess/Plugins/Netscape/mac/NetscapePluginMac.mm:
        (WebKit::NetscapePlugin::popUpContextMenu):
        Convert the coordinates from the current mouse event to screen coordinates and call
        WKPopupContextMenu.

        (WebKit::NetscapePlugin::platformHandleMouseEvent):
        Update m_currentMouseEvent.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76043 268f45cc-cd09-0410-ab3c-d52691b4dbfc