webrtc: prepare for running under remote_run

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[depot_tools::bot_update]/resources/bot_update.py",
       "--master",
       "tryserver.libyuv",
       "--builder",
       "android_gn",
@@ skipping 105 matching lines @@
       "GYP_CROSSCOMPILE": "1",
       "GYP_DEFINES": "OS=android component=static_library dcheck_always_on=1 fastbuild=1 gomadir='[CACHE]/cipd/goma' target_arch=arm use_goma=1",
       "PATH": "[SLAVE_BUILD]/src/third_party/android_tools/sdk/platform-tools:[SLAVE_BUILD]/src/build/android:%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
-      "[DEPOT_TOOLS]/gn.py",
+      "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
       "--root=[SLAVE_BUILD]/src",
       "gen",
       "//out/Debug",
       "--args=is_debug=true target_os=\"android\" target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
     ],
     "name": "gn"
   },
   {
     "cmd": [
       "python",
@@ skipping 34 matching lines @@
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]