/usr/share/gconf/schemas/system_smb.schemas is in libgnomevfs2-common 1:2.24.4-1ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 | <?xml version="1.0"?>
<gconfschemafile>
  <schemalist>
    <schema>
      <key>/schemas/system/smb/workgroup</key>
      <applyto>/system/smb/workgroup</applyto>
      <owner>gnome-vfs</owner>
      <type>string</type>
      <default></default>
      <gettext_domain>gnome-vfs-2.0</gettext_domain>
      <locale name="C">
        <short>SMB workgroup</short>
        <long>The Windows networking workgroup or domain that the user 
        is part of. In order for a new workgroup to fully take effect 
        the user may need to log out and log back in. </long>
      </locale>
    </schema>
  </schemalist>
</gconfschemafile>
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