Reland [Speed Waterfall] Official benchmarks no longer need to run across all platforms.

This is a reland of https://chromium-review.googlesource.com/c/chromium/src/+/1610484

It can be safely landed after
1. PRESUBMIT for issues is fixed
https://chromium-review.googlesource.com/c/chromium/src/+/1612049
2. bot_platforms only include sharded_tests
https://chromium-review.googlesource.com/c/chromium/src/+/1612359

TBR'ing since this is a clean reland of code that was already reviewed.

TBR=jbudorick@chromium.org

Bug: 961830
Change-Id: Idb5546958d57b1499d9410ec3ad60a3ad610f9cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610984
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659774}