FTP: fix directory listing parser for ftp.symantec.com.

Some servers do not send the symlink target that we were expecting,
possibly for security reasons.

BUG=56972
TEST=net_unittests

Review URL: http://codereview.chromium.org/3416028

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@60763 0039d316-1c4b-4281-b951-d872f2087c98