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

scripts/slave/recipes/chromium.expected/full_chromium_fyi_WebKit_Mac___WPTServe.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]/WebKit_Mac___WPTServe",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 182 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe/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 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"
   },
   {
     "cmd": [
       "python",
@@ skipping 76 matching lines @@
       "--config-file",
       "[BUILDER_CACHE]/WebKit_Mac___WPTServe/src/tools/mb/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "browser_tests\n",
       "//out/Release"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@browser_tests@@@",
       "@@@STEP_LOG_END@swarming-targets-file.txt@@@"
     ]
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[BUILDER_CACHE]/WebKit_Mac___WPTServe/src",
       "--output-json",
       "/path/to/tmp/json"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "clang_revision",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"clang_revision\": \"123456-7\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@"
     ]
   },
@@ skipping 33 matching lines @@
       "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--ninja-ensure-up-to-date",
       "--",
       "blink_tests",
       "browser_tests",
       "browser_tests_run"
     ],
     "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)@@@"
     ]
   },
   {
     "cmd": [
@@ skipping 33 matching lines @@
       "python",
       "-u",
       "RECIPE_MODULE[build::isolate]/resources/find_isolated_tests.py",
       "--build-dir",
       "[BUILDER_CACHE]/WebKit_Mac___WPTServe/src/out/Release",
       "--output-json",
       "/path/to/tmp/json"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "find isolated tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"dummy_target_1\": \"[dummy hash for dummy_target_1]\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"dummy_target_2\": \"[dummy hash for dummy_target_2]\"@@@",
       "@@@STEP_LOG_LINE@json.output@}@@@",
       "@@@STEP_LOG_END@json.output@@@",
       "@@@SET_BUILD_PROPERTY@swarm_hashes@{\"dummy_target_1\": \"[dummy hash for dummy_target_1]\", \"dummy_target_2\": \"[dummy hash for dummy_target_2]\"}@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys\nprint '*.isolated file for target %s is missing' % sys.argv[1]\nsys.exit(1)\n",
       "browser_tests"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "[error] browser_tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@print '*.isolated file for target %s is missing' % sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@sys.exit(1)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
@@ skipping 28 matching lines @@
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Mac - WPTServe",
       "--step-name",
       "webkit_tests",
       "--enable-wptserve"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "webkit_tests",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br/>Total executed: 9001<br/>@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/chromium/archive_layout_test_results.py",
       "--results-dir",
       "[SLAVE_BUILD]/layout-test-results",
       "--build-dir",
       "[BUILDER_CACHE]/WebKit_Mac___WPTServe/src/out",
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Mac - WPTServe",
       "--gs-bucket",
       "gs://chromium-layout-test-archives",
       "--staging-dir",
       "[CACHE]/chrome_staging"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "archive_webkit_tests_results",
     "~followup_annotations": [
       "@@@STEP_LINK@layout_test_results@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Mac_-_WPTServe/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Mac_-_WPTServe/571/layout-test-results.zip@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "\nimport sys\nprint '%s wasn\\'t triggered' % sys.argv[1]\nsys.exit(1)\n",
       "browser_tests"
     ],
     "cwd": "[BUILDER_CACHE]/WebKit_Mac___WPTServe",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "[collect error] browser_tests",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@python.inline@@@@",
       "@@@STEP_LOG_LINE@python.inline@import sys@@@",
       "@@@STEP_LOG_LINE@python.inline@print '%s wasn\\'t triggered' % sys.argv[1]@@@",
       "@@@STEP_LOG_LINE@python.inline@sys.exit(1)@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]