[autotest] higher_is_better in tko_iteration_perf_value allows null

Allow higher_is_better to be set to None when sending
perf data to dashboard. Perf dashboard will automatically
detect whether "higher is better" or "lower is better" using
its defaults for the units.

TEST=Run a perf test that outputs higher_is_better=None
sync up: migrate.py sync
sync down: migrate.py sync 96
DEPLOY= migrate
BUG=None

Change-Id: I51502525661d369b14b8f5667ad12d4316db0d41
Reviewed-on: https://chromium-review.googlesource.com/228780
Reviewed-by: Ilja Friedel <ihf@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
1 file changed