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

scripts/slave/recipes/chromium.expected/full_chromium_webkit_WebKit_Mac10_10.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 182 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 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 99 matching lines @@
       "python",
       "-u",
       "RECIPE_MODULE[build::isolate]/resources/find_isolated_tests.py",
       "--build-dir",
       "[BUILDER_CACHE]/mac_layout/src/out/Release",
       "--output-json",
       "/path/to/tmp/json"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "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]\"}@@@"
@@ skipping 57 matching lines @@
       "slavename:TestSlavename",
       "--tag",
       "stepname:browser_tests on Mac-10.10",
       "--idempotent",
       "ffffffffffffffffffffffffffffffffffffffff",
       "--",
       "--test-launcher-summary-output=${ISOLATED_OUTDIR}/output.json"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "[trigger] browser_tests on Mac-10.10",
     "~followup_annotations": [
       "@@@STEP_LOG_LINE@json.output@{@@@",
       "@@@STEP_LOG_LINE@json.output@  \"base_task_name\": \"browser_tests/Mac-10.10/ffffffffff/WebKit Mac10.10/571\", @@@",
       "@@@STEP_LOG_LINE@json.output@  \"tasks\": {@@@",
       "@@@STEP_LOG_LINE@json.output@    \"browser_tests/Mac-10.10/ffffffffff/WebKit Mac10.10/571\": {@@@",
       "@@@STEP_LOG_LINE@json.output@      \"shard_index\": 0, @@@",
       "@@@STEP_LOG_LINE@json.output@      \"task_id\": \"10000\", @@@",
@@ skipping 34 matching lines @@
       "test-results.appspot.com",
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Mac10.10",
       "--step-name",
       "webkit_tests"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "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]/mac_layout/src/out",
       "--build-number",
       "571",
       "--builder-name",
       "WebKit Mac10.10",
       "--gs-bucket",
       "gs://chromium-layout-test-archives",
       "--staging-dir",
       "[CACHE]/chrome_staging",
       "--gs-acl",
       "public-read"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "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_Mac10_10/571/layout-test-results/results.html@@@",
       "@@@STEP_LINK@(zip)@https://storage.googleapis.com/chromium-layout-test-archives/WebKit_Mac10_10/571/layout-test-results.zip@@@"
     ]
   },
   {
     "allow_subannotations": true,
@@ skipping 13 matching lines @@
       "collect",
       "--swarming",
       "https://chromium-swarm.appspot.com",
       "--decorate",
       "--print-status-updates",
       "--json",
       "{\"base_task_name\": \"browser_tests/Mac-10.10/ffffffffff/WebKit Mac10.10/571\", \"tasks\": {\"browser_tests/Mac-10.10/ffffffffff/WebKit Mac10.10/571\": {\"shard_index\": 0, \"task_id\": \"10000\", \"view_url\": \"https://chromium-swarm.appspot.com/user/task/10000\"}}}"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "browser_tests on Mac-10.10",
     "~followup_annotations": [
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::test_results]/resources/upload_test_results.py",
       "--input-json",
       "{\"per_iteration_data\": [{\"Test.One\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}], \"Test.Three\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}], \"Test.Two\": [{\"elapsed_time_ms\": 0, \"output_snippet\": \"\", \"status\": \"SUCCESS\"}]}], \"swarming_summary\": {\"shards\": [{\"outputs_ref\": {\"view_url\": \"blah\"}}]}}",
       "--master-name",
       "chromium.webkit",
       "--builder-name",
       "WebKit Mac10.10",
       "--build-number",
       "571",
       "--test-type",
       "browser_tests on Mac-10.10",
       "--test-results-server",
       "test-results.appspot.com",
       "--chrome-revision",
       "170242"
     ],
     "cwd": "[BUILDER_CACHE]/mac_layout",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "Upload to test-results [browser_tests on Mac-10.10]"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]