Update GCE bots to Debian 9.2

Bug: skia:
Change-Id: Ie56f848889cfc7331109aed997a85bc42e27f60e
Reviewed-on: https://skia-review.googlesource.com/70724
Commit-Queue: Eric Boren <borenet@google.com>
Reviewed-by: Ben Wagner <benjaminwagner@google.com>
2 files changed