chromium.run_mb: set cwd to checkout

scripts/slave/recipes/ios/try.expected/gn.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 259 matching lines @@
       "-m",
       "chromium.mac",
       "-b",
       "ios-simulator-gn",
       "--config-file",
       "[SLAVE_BUILD]/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "//out/Debug-iphonesimulator"
     ],
+    "cwd": "[SLAVE_BUILD]/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files (with patch)"
   },
   {
     "cmd": [
       "git",
       "diff",
       "--cached",
@@ skipping 80 matching lines @@
     "~followup_annotations": [
       "@@@STEP_TEXT@This test was skipped because it was not affected.@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]