chromium: run GN in the checkout directory

scripts/slave/recipes/libyuv/libyuv.expected/client_libyuv_linux64_debug__gn_.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "client.libyuv",
       "--builder",
       "Linux64 Debug (GN)",
@@ skipping 108 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Debug",
       "--args=is_debug=true target_cpu=\"x64\" 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 26 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]