chromium: run GN in the checkout directory

scripts/slave/recipes/webrtc/standalone.expected/client_webrtc_mac64_release__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 182 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[BUILDER_CACHE]/Mac64_Release__GN_/src",
       "gen",
       "//out/Release",
       "--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]/Mac64_Release__GN_/src",
     "name": "gn"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[BUILDER_CACHE]/Mac64_Release__GN_/src",
@@ skipping 50 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]