Perf Tests: Divide test samples between shards. Instead of a single test shard running all samples for a test, we run a single sample on different shards to produce a combined measurement that can account for variance between test machines. Bug: angleproject:7671 Change-Id: I04bbd51de257f2f299783ce3bd65da3d9348c8dc Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3937044 Reviewed-by: Yuly Novikov <ynovikov@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>