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This is a reland of 9c270678

Fixed invalid unit and improved the error message for an invalid unit.

`./tools/perf/run_benchmark run webrtc --browser=system` passes locally
on Mac.

Original change's description:
> Mark some WebRTC perf measurements as biggerIsBetter
>
> Change-Id: I95a145f0380b0f413a71ef5a9bbacf840eb20b9f
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577205
> Commit-Queue: John Chen <johnchen@chromium.org>
> Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
> Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
> Reviewed-by: John Chen <johnchen@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#834889}

Change-Id: Id7220dafb11faab2cb3e28b36aed3c6a526575c5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580072Reviewed-by: Mirko Bonadei <mbonadei@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Auto-Submit: Ben Wagner aka dogben <benjaminwagner@google.com>
Cr-Commit-Position: refs/heads/master@{#835200}