Add OS to SkiaCT dimensions.

This will make it much easier to experiment with upgrading the OS to
Debian-9.1 without causing failures on master.

No-Try: true
Change-Id: Id43b055841ec3ceb42133c9dd7b630f12d1b45c6
Reviewed-on: https://skia-review.googlesource.com/31001
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
2 files changed