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This is useful for me as I play with getting sharding working for abridged benchmarks.
Also update help docs so that there is one help doc for each subcommand to make the
--help output look nicer.

Bug: 965158
Change-Id: Ia30f029902bff8c5fea60a1dcf70266c8fc78dd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884879
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710525}