[snapshots] Update Snapshot Builder to Xenial

Bug: 966276
Change-Id: I347386b75c3f595050248f33993fc48f893ef2bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626808Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: smut <smut@google.com>
Cr-Commit-Position: refs/heads/master@{#662847}

infra/config/cr-buildbucket-dev.cfg

@@ -103,7 +103,7 @@ buckets {
 
       builders {
         name: "Snapshot Builder"
-        dimensions: "os:Ubuntu-14.04"
+        dimensions: "os:Ubuntu-16.04"
         execution_timeout_secs: 3600
         service_account: "snapshot-builder@chops-service-accounts.iam.gserviceaccount.com"
         recipe {