Better variable naming in SwViewImp::NotifySizeChg

Make it obvious, which variable contains the document boundaries and
which the draw / fly object boundaries.

Change-Id: I0bc2bcb5515e317c7a204b3458b92c61482e1a5c
Reviewed-on: https://gerrit.libreoffice.org/10974
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>