[infra] Specify machine_type dimension for Linux GCE bots.

This is in preparation of assigning bots either "small" or "large" GCE
machines.

Bug: skia:7415, skia:7416
Change-Id: I82b6fe454ba3622ef8a9d515dc3e72362dd30ee5
Reviewed-on: https://skia-review.googlesource.com/124183
Commit-Queue: Eric Boren <borenet@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed