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This introduces an experimental test suite that shards junit tests
to run faster.

This is to land after junit sharding lands:
https://chromium-review.googlesource.com/c/chromium/src/+/2490847

Bug: 1138506
Change-Id: I01e38a2e80686383e48908110815551ac5526e72
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493822
Commit-Queue: benjamin joyce <bjoyce@chromium.org>
Reviewed-by: Peter Wen <wnwen@chromium.org>
Reviewed-by: Haiyang Pan <hypan@google.com>
Cr-Commit-Position: refs/heads/master@{#823770}