Make all delta payloads optional. This change makes the nton payload optional in addition to the mton payload which was done in Ib6704594806de33cea38beb968e30304f1529211. BUG=chromium-os:29567 TEST=unittests + downloader using a lumpy cq build: unified-lumpy-paladin/R21-2343.0.0-rc8 Change-Id: I7b107b5b055cd78b5210400950c8fa388f6d4f7b Reviewed-on: https://gerrit.chromium.org/gerrit/23300 Reviewed-by: Chris Sosa <sosa@chromium.org> Tested-by: Chris Sosa <sosa@chromium.org> Commit-Ready: Chris Sosa <sosa@chromium.org>