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

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Tester.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_Tester",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 180 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "GYP_DEFINES": "clang=1 component=static_library ffmpeg_branding=Chrome gomadir='[CACHE]/cipd/goma' proprietary_codecs=1 target_arch=x64 use_goma=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 106 matching lines @@
       "--slave-name=TestSlavename",
       "--build-number=571",
       "content_browsertests",
       "--gtest_filter=WebRtc*",
       "--run-manual",
       "--test-launcher-print-test-stdio=always",
       "--test-launcher-bot-mode"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "content_browsertests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
@@ skipping 17 matching lines @@
       "browser_tests",
       "--gtest_filter=WebRtc*:Webrtc*:TabCapture*:*MediaStream*",
       "--run-manual",
       "--ui-test-action-max-timeout=350000",
       "--test-launcher-jobs=1",
       "--test-launcher-bot-mode",
       "--test-launcher-print-test-stdio=always"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "browser_tests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
@@ skipping 12 matching lines @@
       "--slave-name=TestSlavename",
       "--build-number=571",
       "capture_unittests",
       "--enable-logging",
       "--v=1",
       "--test-launcher-jobs=1",
       "--test-launcher-print-test-stdio=always"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "capture_unittests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"Mac Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_build_archive_url\": \"gs://test-domain/test-archive.zip\", \"parent_buildername\": \"Mac Builder\", \"parent_got_revision\": \"1111111\", \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"Mac Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_build_archive_url\": \"gs://test-domain/test-archive.zip\", \"parent_buildername\": \"Mac Builder\", \"parent_got_revision\": \"1111111\", \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--annotate=gtest",
       "--test-type=content_unittests",
       "--step-name=content_unittests",
       "--test-launcher-summary-output",
       "/path/to/tmp/json",
       "--builder-name=Mac Tester",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "content_unittests"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "content_unittests"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/runtest.py",
       "--target",
       "Release",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"Mac Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_build_archive_url\": \"gs://test-domain/test-archive.zip\", \"parent_buildername\": \"Mac Builder\", \"parent_got_revision\": \"1111111\", \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"build_data_dir\": \"[ROOT]/build_data_dir\", \"buildername\": \"Mac Tester\", \"buildnumber\": 571, \"mastername\": \"chromium.webrtc.fyi\", \"parent_build_archive_url\": \"gs://test-domain/test-archive.zip\", \"parent_buildername\": \"Mac Builder\", \"parent_got_revision\": \"1111111\", \"path_config\": \"kitchen\", \"recipe\": \"chromium\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--annotate=gtest",
       "--test-type=remoting_unittests",
       "--step-name=remoting_unittests",
       "--test-launcher-summary-output",
       "/path/to/tmp/json",
       "--builder-name=Mac Tester",
       "--slave-name=TestSlavename",
       "--build-number=571",
       "remoting_unittests",
       "--gtest_filter=Webrtc*"
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Tester",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "remoting_unittests"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]