Download the hermetic xcode toolchain on all bots that target mac.

scripts/slave/recipes/chromium_trybot.expected/use_v8_patch_on_blink_trybot.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]/mac_layout",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 191 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout/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=static_library dcheck_always_on=1 fastbuild=1 gomadir='[CACHE]/cipd/goma' target_arch=x64 test_isolation_mode=prepare use_goma=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks (with patch)"
   },
   {
     "cmd": [
       "python",
@@ skipping 117 matching lines @@
       "import sys; sys.exit(0)"
     ],
     "name": "mark: before_tests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]