Add an experimental cros-dut-flash builder to buildbucket-dev.

Its bots will be everything in:
https://chromium-swarm-dev.appspot.com/botlist?c=id&c=os&c=task&c=status&f=os%3AChromeOS&l=100&q=os%3AChr&s=id%3Aasc

Bug: 901205
Change-Id: I0266b1c75deacdd0b287bcd9cc7f8b247a4792af
Reviewed-on: https://chromium-review.googlesource.com/c/1313836Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604984}

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

@@ -87,6 +87,14 @@ buckets {
       dimensions: "os:Windows-10"
       mixins: "swarm-ci"
     }
+    # TODO(crbug.com/901205): Move these to prod.
+    builders {
+      # An experimental builder that runs a recipe used to flash its DUT.
+      name: "cros-dut-flash"
+      recipe {
+        name: "cros_flash"
+      }
+    }
   }
 }