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These are called WindowsServer2008 and Windows-2012ServerR2-SP0 and
stuff like that nowadays. This CL just drops the OS dimension since
bots.cfg ensures each builder runs on the right OS, anyway.

Tbr: jbudorick@chromium.org
Bug: 877018
Change-Id: Ib71ee5f1d26ed90b38fb0b9868994f4bd0c29ad3
Reviewed-on: https://chromium-review.googlesource.com/c/1354924Reviewed-by: Patrik Höglund <phoglund@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Patrik Höglund <phoglund@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612183}