Avoid to use environment.

scripts/slave/recipes/chromium.expected/full_chromium_webkit_WebKit_Mac10_9.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys, os\npath = sys.argv[1]\nmode = int(sys.argv[2])\nif not os.path.isdir(path):\n  if os.path.exists(path):\n    print \"%s exists but is not a dir\" % path\n    sys.exit(1)\n  os.makedirs(path, mode)\n",
       "[BUILDER_CACHE]/mac_layout",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 306 matching lines @@
       "--gsutil-py-path",
       "RECIPE_PACKAGE_REPO[depot_tools]/gsutil.py",
       "--ninja-path",
       "RECIPE_PACKAGE_REPO[depot_tools]/ninja",
       "--target",
       "Release",
       "--src-dir",
       "[BUILDER_CACHE]/mac_layout/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
+      "--buildbot-buildername",
+      "WebKit Mac10.9",
+      "--buildbot-mastername",
+      "chromium.webkit",
+      "--buildbot-slavename",
+      "TestSlavename",
       "--goma-deps-cache-dir",
       "[GOMA_DEPS_CACHE]",
       "--build-data-dir",
       "[ROOT]/build_data_dir",
       "--compiler",
       "goma-clang",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
       "--goma-service-account-json-file",
       "/creds/service_accounts/service-account-goma-client.json",
@@ skipping 113 matching lines @@
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Mac10_9/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Mac10_9/571/layout-test-results.zip@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]