Use xenial explicitly for builder

This CL completely migrates dawn linux builder from trusty to xenial.

BUG=chromium:981741
Change-Id: I5e9bdba7bd58355acaaeb9eebb120155b7996fd7
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/9060
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
diff --git a/infra/config/global/cr-buildbucket.cfg b/infra/config/global/cr-buildbucket.cfg
index 88b541b..515e804 100644
--- a/infra/config/global/cr-buildbucket.cfg
+++ b/infra/config/global/cr-buildbucket.cfg
@@ -104,25 +104,25 @@
     # Linux: test combinations of {clang}x{release,debug}x{x86,x64}
     builders {
       name: "linux-clang-dbg-x64"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "debug"
       mixins: "x64"
     }
     builders {
       name: "linux-clang-dbg-x86"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "debug"
       mixins: "x86"
     }
     builders {
       name: "linux-clang-rel-x64"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "release"
       mixins: "x64"
     }
     builders {
       name: "linux-clang-rel-x86"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "release"
       mixins: "x86"
     }
@@ -212,7 +212,7 @@
 
     builders {
       name: "presubmit"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       recipe {
         name: "run_presubmit"
         properties: "repo_name:dawn"
@@ -227,28 +227,28 @@
     # CI Mirror only
     builders {
       name: "linux-clang-dbg-x64"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "debug"
       mixins: "x64"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-dbg-x86"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "debug"
       mixins: "x86"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-rel-x64"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "release"
       mixins: "x64"
     }
     # Actually on the CQ
     builders {
       name: "linux-clang-rel-x86"
-      dimensions: "os:Ubuntu"
+      dimensions: "os:Ubuntu-16.04"
       mixins: "release"
       mixins: "x86"
     }