Respect the metrics file if no policy value is set (for legacy migration).

BUG=chromium-os:18430
TEST=Update chromeos and check whether the consent for stats is preserved.

Change-Id: I86d4710ef9f381abd9f1eac728040878e6fc5b71
Reviewed-on: http://gerrit.chromium.org/gerrit/5139


Reviewed-by: Ken Mixter <kmixter@chromium.org>
Tested-by: Julian Pastarmov <pastarmovj@chromium.org>