Migrate Linux CQ from Xenial to Bionic

BUG=chromium:1212166

Change-Id: I605fd926e0235da4f8e5b60bd471971dc1bded32
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/52021
Auto-Submit: Ryan Harrison <rharrison@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

infra/config/global/cr-buildbucket.cfg

diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index 22354e0..47accfa 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -72,7 +72,7 @@
 }
 builder_mixins {
   name: "linux",
-  dimensions: "os:Ubuntu-16.04"
+  dimensions: "os:Ubuntu-18.04"
   recipe {
     properties_j: <<EOF
       $build/goma: {
@@ -265,7 +265,7 @@
 
     builders {
       name: "presubmit"
-      dimensions: "os:Ubuntu-16.04"
+      dimensions: "os:Ubuntu-18.04"
       recipe {
         name: "run_presubmit"
         properties: "repo_name:tint"