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This is a reland of fab66bae

Includes necessary change to luci-scheduler.cfg and updated docs.

Original change's description:
> Add new ANGLE Linux and Mac tester configs.
>
> These configs will replace linux_angle_rel_ng and mac_angle_rel_ng.
> They initially mirror the configuration but will soon be updated to
> run only the subsets of tests that are most relevant to ANGLE. This
> should reduce flakiness in ANGLE and reduce load for Chrome.
>
> Also updates related configuration files to include linux-angle-rel
> and mac-angle-rel.
>
> Bug: 822310
> Change-Id: I2b40ed45f5d9e105c088ab166bd2207c96280151
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1551463
> Reviewed-by: John Budorick <jbudorick@chromium.org>
> Reviewed-by: Kenneth Russell <kbr@chromium.org>
> Commit-Queue: Jamie Madill <jmadill@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#647854}

Bug: 822310
Tbr: jbudorick@chromium.org
Change-Id: Iaeeffc7b089b05d379bb353d627d13b39fd43704
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1554296
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648727}