infra: ci.star chromiumos_builder() passes **kwargs

Previously, chromiumos_builder() ignored the **kwargs argument.

This was a problem in http://crrev.com/c/1923225, when the goma_backend
arg was not being passed to builder(). This resulted in some builders
not actually getting the arg to use Goma RBE backend:
- linux-chromeos-dbg
- linux-chromeos-rel

With this CL, these builders will get the correct goma_backend setting,
and start building using the Goma RBE backend.

Bug: 950413
Change-Id: Ie389646206ec96e39f1ad436db341ef213b539c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1924182
Commit-Queue: Simon Que <sque@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716897}

infra/config/buckets/ci.star

@@ -442,6 +442,7 @@ def chromiumos_builder(*, name, **kwargs):
   return builder(
       name = name,
       mastername = 'chromium.chromiumos',
+      **kwargs
   )
 
 chromiumos_builder(

infra/config/generated/cr-buildbucket.cfg

@@ -7047,6 +7047,7 @@ buckets: <
         name: "chromium"
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"
+        properties_j: "$build/goma:{\"rpc_extra_params\":\"?prod\",\"server_host\":\"goma.chromium.org\"}"
         properties_j: "$kitchen:{\"devshell\":true,\"git_auth\":true}"
         properties_j: "mastername:\"chromium.chromiumos\""
       >
@@ -7067,6 +7068,7 @@ buckets: <
         name: "chromium"
         cipd_package: "infra/recipe_bundles/chromium.googlesource.com/chromium/tools/build"
         cipd_version: "refs/heads/master"
+        properties_j: "$build/goma:{\"rpc_extra_params\":\"?prod\",\"server_host\":\"goma.chromium.org\"}"
         properties_j: "$kitchen:{\"devshell\":true,\"git_auth\":true}"
         properties_j: "mastername:\"chromium.chromiumos\""
       >