Run nanobench on perf bots.

We have a new harness, "nanobench", for our microbenchmarks that we trust more than the old "bench".  I figure it's time to get the bots running it.

Plan:
  1) land this
  2) see what bots if any needs special consideration; fix them
  3) update upload_bench_results.py to look for nanobench_ files instead of microbench_ files.
  4) clean up RunBench

BUG=skia:
R=borenet@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/393593003