chromium: run GN in the checkout directory

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_win64_release__gn_.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]\\Win64_Release__GN_",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 160 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]\\gn.py",
       "--root=[BUILDER_CACHE]\\Win64_Release__GN_\\src",
       "gen",
       "//out/Release_x64",
       "--args=ffmpeg_branding=\"Chrome\" rtc_use_h264=true is_debug=false target_cpu=\"x64\" use_goma=true goma_dir=\"[CACHE]\\cipd\\goma\""
     ],
+    "cwd": "[BUILDER_CACHE]\\Win64_Release__GN_\\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 28 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]