V8: Run MB on staging bot.

scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json

Index: scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json
diff --git a/scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json b/scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json
index d4805fb15c1b7cf6130cbee21208d33dc64873a5..a716da7e0fbe94e6f0bcad9eddf8790c8aae7abc 100644
--- a/scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json
+++ b/scripts/slave/recipes/v8.expected/full_client_v8_V8_Linux___swarming_staging.json
@@ -163,13 +163,23 @@
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
-      "--root=[SLAVE_BUILD]/v8",
+      "[SLAVE_BUILD]/v8/tools/mb/mb.py",
       "gen",
-      "//out/Release",
-      "--args=use_sysroot=false icu_use_data_file = false v8_test_isolation_mode = \"prepare\" is_debug=false target_cpu=\"x64\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+      "-m",
+      "client.v8",
+      "-b",
+      "V8 Linux - swarming staging",
+      "--config-file",
+      "[SLAVE_BUILD]/v8/infra/mb/mb_config.pyl",
+      "--goma-dir",
+      "[CACHE]/cipd/goma",
+      "--gyp-script=gypfiles/gyp_v8",
+      "//out/Release"
     ],
-    "name": "gn"
+    "env": {
+      "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
+    },
+    "name": "generate_build_files"
   },
   {
     "allow_subannotations": true,