Reviewed by Xan Lopez.

        Fix crash when downloading, caused by assuming SoupMessage would
        be there. This change factors the logic required to create a
        ResourceRequest from a WebKitNetworkRequest into a core()
        function, like we have for some other classes.

        * webkit/webkitdownload.cpp:
        (webkit_download_start):
        * webkit/webkitprivate.cpp:
        (WebKit::core):
        * webkit/webkitprivate.h:
        * webkit/webkitwebframe.cpp:
        (webkit_web_frame_load_request):

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@44619 268f45cc-cd09-0410-ab3c-d52691b4dbfc