chromium: run GN in the checkout directory

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.libyuv",
       "--builder",
       "android_gn_rel",
@@ skipping 111 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Release",
       "--args=is_debug=false 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 29 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]