Make private browsing work on Windows.
https://bugs.webkit.org/show_bug.cgi?id=106032.

Reviewed by Alexey Proskuryakov.

Code path never called ensurePrivateBrowsingSession which is what starts up the private browsing session.
This was causing many tests to involving private browsing to fail.

* WebView.cpp:
(WebView::notifyPreferencesChanged):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@138865 268f45cc-cd09-0410-ab3c-d52691b4dbfc