chromium.run_mb: set cwd to checkout

scripts/slave/recipes/chromium.expected/full_chromium_mac_Mac_Builder.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",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 301 matching lines @@
       "-b",
       "Mac Builder",
       "--config-file",
       "[BUILDER_CACHE]/mac/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "browser_tests\n",
       "//out/Release"
     ],
+    "cwd": "[BUILDER_CACHE]/mac/src",
     "env": {
       "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@browser_tests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
@@ skipping 194 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@Mac10.10 Tests<br />Mac10.11 Tests<br />Mac10.9 Tests@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]