Added NetworkInterface::network_prefix.

BUG=295997
NOTRY=true

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=225303

R=szym@chromium.org

Review URL: https://codereview.chromium.org/23726043

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225353 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225353