Add os:ChromeOS dimension to experimental buildbucket cros-flash builder.

And remove cpu:x64 from the defaults.

TBR=jbudorick

Bug: 901205
Change-Id: I64be7685d85a2b5d895dafde6df0193b8db0c6bf
Reviewed-on: https://chromium-review.googlesource.com/c/1315693Reviewed-by: Ben Pastene <bpastene@chromium.org>
Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605001}

infra/config/global/cr-buildbucket-dev.cfg

@@ -21,6 +21,7 @@ acl_sets {
 
 builder_mixins {
   name: "swarm-ci"
+  dimensions: "cpu:x86-64"
   recipe {
     name: "swarming/staging"
     properties: "mastername:chromium.swarm"
@@ -47,7 +48,6 @@ buckets {
 
     builder_defaults {
       category: "Chromium"
-      dimensions: "cpu:x86-64"
       execution_timeout_secs: 10800  # 3h
       service_account: "chromium-ci-builder-dev@chops-service-accounts.iam.gserviceaccount.com"
       build_numbers: YES
@@ -91,6 +91,7 @@ buckets {
     builders {
       # An experimental builder that runs a recipe used to flash its DUT.
       name: "cros-dut-flash"
+      dimensions: "os:ChromeOS"
       recipe {
         name: "cros_flash"
       }