See example v8_linux64_perf_try(_triggered) builds on this CL triggered using
the following command lines:

  python tools/try_perf.py --linux64 --confidence-level 1 compile
  python tools/try_perf.py --linux64 --confidence-level 1 arewefastyet
  python tools/try_perf.py --linux64 --confidence-level 3 arewefastyet

This also fixes running tools/run_perf.py --help and adds logging for the
current confidence level, which allows users to monitor progress. Example runs:

  https://chrome-swarming.appspot.com/task?id=456e4d6e743cc510 (Compile)
  https://chrome-swarming.appspot.com/task?id=456e5145615aa510 (JetStream)
  https://chrome-swarming.appspot.com/task?id=456e53eeb9104410 (JSBench)
  https://chrome-swarming.appspot.com/task?id=456e541e0e13bc10 (AreWeFastYet)

Finally, this adds support for fractional confidence levels. Example runs:

  https://chrome-swarming.appspot.com/task?id=456e5970e6f24410 (AreWeFastYet)
  https://chrome-swarming.appspot.com/task?id=456e5a8f3f407c10 (Compile)

R=tmrts@chromium.org, machenbach@chromium.org

Bug: chromium:880724
Change-Id: I725a83060c0bdd3ef08a7f0e4df843611c712d37
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1660471Reviewed-by: Tamer Tas <tmrts@chromium.org>
Commit-Queue: Sergiy Belozorov <sergiyb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62176}