BUILD FIX (r150049): Fix RetainPtr<> type in PasteboardIOS.mm

* platform/ios/PasteboardIOS.mm:
(WebCore::Pasteboard::writeImage): Use RetainPtr<CFStringRef>
instead of RetainPtr<NSString>.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@150054 268f45cc-cd09-0410-ab3c-d52691b4dbfc