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This reverts commit 99255887.

Reason for revert: added more machines, going to try out warm caching

Original change's description:
> cr-buildbucket-dev: use auto_builder_dimension for luci.chromium.ci.
> 
> Bug: 868525
> Change-Id: I65aabb60d4823423d7f0bd4d5b51ea145c665658
> Reviewed-on: https://chromium-review.googlesource.com/1176471
> Reviewed-by: Nodir Turakulov <nodir@chromium.org>
> Commit-Queue: John Budorick <jbudorick@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#583450}

TBR=nodir@chromium.org,jbudorick@chromium.org

Change-Id: If807d179cf51089c8332a627ae4d2de78468e79a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 868525
Reviewed-on: https://chromium-review.googlesource.com/1178993Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: John Budorick <jbudorick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583856}