libyuv: Switch bots over to GN by default.

scripts/slave/recipes/libyuv/libyuv.expected/client_libyuv_mac64_debug__gyp_.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 111 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[SLAVE_BUILD]/src",
     "env": {
       "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
-      "GYP_CHROMIUM_NO_ACTION": "1",
       "GYP_DEFINES": "clang=1 component=shared_library gomadir='[CACHE]/cipd/goma' target_arch=x64 use_goma=1",
       "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",
-      "--args=is_debug=true target_cpu=\"x64\" 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 21 matching lines @@
       "RECIPE_PACKAGE_REPO[depot_tools]/gsutil.py",
       "--ninja-path",
       "RECIPE_PACKAGE_REPO[depot_tools]/ninja",
       "--target",
       "Debug",
       "--src-dir",
       "[SLAVE_BUILD]/src",
       "--goma-cache-dir",
       "[GOMA_CACHE]",
       "--buildbot-buildername",
-      "Mac64 Debug (GN)",
+      "Mac64 Debug (GYP)",
       "--buildbot-mastername",
       "client.libyuv",
       "--buildbot-slavename",
       "slavename",
       "--goma-deps-cache-file",
-      "Mac64_Debug__GN_.gomadeps",
+      "Mac64_Debug__GYP_.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": {
       "FORCE_MAC_TOOLCHAIN": "1",
       "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
   }
 ]