[infra] Add image dimension for Win GCE bots

Change-Id: Ib7a8205992c6c1f4d93448afe54468f77511f1f0
Reviewed-on: https://skia-review.googlesource.com/123585
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
2 files changed