[WinCairo] Build fails.
https://bugs.webkit.org/show_bug.cgi?id=122830

Patch by peavo@outlook.com <peavo@outlook.com> on 2013-10-15
Reviewed by Brent Fulgham.

* platform/network/NetworkStorageSessionStub.cpp:
(WebCore::NetworkStorageSession::createPrivateBrowsingSession): Update to new return type.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@157450 268f45cc-cd09-0410-ab3c-d52691b4dbfc