related to #fdo71248 - ActiveDocument not tracking ThisWordDoc

Making the same change that fixed ThisExcelDoc in 2012
(commit cea414bfe8fe356793778bdf09f5ca3b3a42daa0),
but matching the current code structure
(commit 191bf98991e1934bfef115a1cec463ac250b0191)

Change-Id: I0a5c1a006d5a73b8e3be1b6b407e62927d49c36a
Reviewed-on: https://gerrit.libreoffice.org/11842
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>