Switch test server to new-style "device gone" status code.

BUG=None
TEST=compiles and passes tests


Review URL: http://codereview.chromium.org/7200026

git-svn-id: http://src.chromium.org/svn/trunk/src/chrome/browser/policy/proto@89827 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
diff --git a/device_management_backend.proto b/device_management_backend.proto
index 857db34..690fb87 100644
--- a/device_management_backend.proto
+++ b/device_management_backend.proto
@@ -246,6 +246,7 @@
 // 401 Unauthorized: invalid auth cookie or DM token.
 // 403 Forbidden: device management is not allowed.
 // 404 Not Found: the request URL is invalid.
+// 410 Gone: device is unknown to the server.
 // 491 Request Pending: the request is pending approval.
 // 500 Internal Server Error: most likely a bug in DM server.
 // 503 Service Unavailable: most likely a backend error.