Upgrade Debian9 bots to 9.4.

Cherry-pick to skqp/dev branch.

Change-Id: I912e453ee1f27a5c5e50483d0cfa2ff9862d8e37
Reviewed-On: https://skia-review.googlesource.com/119261
Reviewed-By: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>
Bug: skia:
Reviewed-on: https://skia-review.googlesource.com/119571
2 files changed