Remove private system API calls in dns_config_service_posix on iOS

BUG=335000

Review URL: https://codereview.chromium.org/135043003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245391 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245391