Perf Tests: Enable --split-shard-samples on the bots. This should make all sharts run at most one sample for each test. This in turn should mitigate inter-run variance because we'll sample from a wider pool of machines for each test. Bug: angleproject:7738 Change-Id: Ied96235edff81c8ea6f3fb8ac0e4fb21d6254d4e Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3942604 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>