Add missing refs.cfg.

This is necessary for CQ/Luci operation.

Bug: 833848
Change-Id: Ib42216d3a2ee58e819f42fc9535fa91797f55200
Reviewed-on: https://chromium-review.googlesource.com/c/1332529
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

infra/config/global/refs.cfg

@@ -0,0 +1,6 @@
+# Refs configuration file. The documentation of the format can be found
+# at https://luci-config.appspot.com/schemas/projects:refs.cfg.
+refs {
+  name: "refs/heads/master"
+  config_path: "infra/config/branch"
+}