Browser and Hide Whitespace page height now use
SwPageFrm::GetContentHeight, which is const.

A few improvements are done for both paths as well.

Reviewed-on: https://gerrit.libreoffice.org/19730Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit f05da45c)

Conflicts:
	sw/source/core/layout/calcmove.cxx

Change-Id: I73a8e920ccfa96d76cbbb002bed6a85f2e636ede