Allow specifying machine-type for Swarming VMs.
Allow specifying machine-type for Swarming VMs.

Bug: skia:7409
Change-Id: I21ab86585825de98b8ceea50754fe84efc9bc8d9
Reviewed-on: https://skia-review.googlesource.com/85741
Reviewed-by: Eric Boren <0e499112533c8544f0505ea0d08394fb5ad7d8fa@google.com>
Commit-Queue: Ben Wagner <3ef7217be91069877d94f7907ce5479000772cd3@google.com>

google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot