chromium: run GN in the checkout directory

scripts/slave/recipe_modules/cronet/example.expected/gn_test.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\n\nbuild_path = sys.argv[1]\nif os.path.exists(build_path):\n  for (path, dir, files) in os.walk(build_path):\n    for cur_file in files:\n      if cur_file.endswith('index.lock'):\n        path_to_file = os.path.join(path, cur_file)\n        print 'deleting %s' % path_to_file\n        os.remove(path_to_file)\n",
       "[SLAVE_BUILD]"
     ],
     "name": "cleanup index.lock",
     "~followup_annotations": [
@@ skipping 234 matching lines @@
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/src/buildtools/linux64/gn",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Debug",
       "--args=disable_brotli_filter=true disable_file_support=true disable_ftp_support=true enable_websockets=false use_platform_icu_alternatives=true is_debug=true target_os=\"android\" target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
     ],
+    "cwd": "[SLAVE_BUILD]/src",
     "name": "gn"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/compile.py",
@@ skipping 460 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  os.remove(report)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]