libyuv: Switch bots over to GN by default.

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/tools/runit.py",
       "--show-path",
       "python",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/cleanup_temp.py"
     ],
@@ skipping 110 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
+      "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_CROSSCOMPILE": "1",
       "GYP_DEFINES": "OS=ios clang=1 component=static_library gomadir='[CACHE]/cipd/goma' target_arch=arm use_goma=1",
       "GYP_GENERATORS": "ninja",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
+    "cmd": [
+      "python",
+      "-u",
+      "RECIPE_PACKAGE_REPO[depot_tools]/gn.py",
+      "--root=[SLAVE_BUILD]/src",
+      "gen",
+      "//out/Debug-iphoneos",
+      "--args=is_clang=true target_os=ios is_component_build=false is_debug=true target_cpu=\"arm\" use_goma=true goma_dir=\"[CACHE]/cipd/goma\""
+    ],
+    "cwd": "[SLAVE_BUILD]/src",
+    "name": "gn"
+  },
+  {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--output-json",
       "/path/to/tmp/json"
     ],
@@ skipping 35 matching lines @@
       "--goma-deps-cache-file",
       "iOS_Debug.gomadeps",
       "--compiler",
       "goma-clang",
       "--goma-jsonstatus",
       "/path/to/tmp/json",
       "--goma-service-account-json-file",
       "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[CACHE]/cipd/goma",
-      "--",
-      "All"
+      "--"
     ],
     "env": {
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output (invalid)@null@@@",
       "@@@STEP_LOG_END@json.output (invalid)@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]