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>
2 files changed