[autotest] telemetry_Benchmarks upload perf data using output_perf_value

telemetry_Benchmarks now upload perf data using output_perf_value.
It will not write to the keyval file anymore.

After the change lands, we should stop the scripts extract_perf.py
and generate_perf_graphs that are currently kicked off
by cron to upload the perf data.

Also makes output_perf_value accept empty unit.

TEST=1)Locally kicked off the tests from the AFE,
confirmed that the perf data was uploaded to the testing
perf dashboard.
2)test_unittest.py telemetry_runner_unittest.py

Change-Id: Icf6dca08cab0268b41f8359eb73084dfee3e4b9d
Reviewed-on: https://chromium-review.googlesource.com/176746
Reviewed-by: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
5 files changed