FIxed NetworkInterface::network_prefix calculation on XP.
XP has no OnLinkPrefixLength so we need to use FirstPrefix list instead.

Review URL: https://codereview.chromium.org/25167002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226106 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226106