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

scripts/slave/recipes/chromium.expected/full_chromium_fyi_ClangToTMacASan.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]/ClangToTMacASan",
       "511"
     ],
     "name": "makedirs checkout path",
@@ skipping 124 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/ClangToTMacASan/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": "asan=1 clang=1 clang_use_chrome_plugins=0 component=static_library fastbuild=0 target_arch=x64 test_isolation_mode=prepare",
       "LLVM_FORCE_HEAD_REVISION": "YES",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
@@ skipping 75 matching lines @@
       "-b",
       "ClangToTMacASan",
       "--config-file",
       "[BUILDER_CACHE]/ClangToTMacASan/src/tools/mb/mb_config.pyl",
       "--swarming-targets-file",
       "browser_tests\n",
       "//out/Release"
     ],
     "cwd": "[BUILDER_CACHE]/ClangToTMacASan/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]/ClangToTMacASan/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",
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "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 25 matching lines @@
       "--goma-deps-cache-file",
       "ClangToTMacASan.gomadeps",
       "--build-data-dir",
       "[ROOT]/build_data_dir",
       "--compiler",
       "clang",
       "--",
       "all"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "name": "compile"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILDER_CACHE]/ClangToTMacASan/src/tools/determinism/remove_build_metadata.py",
@@ skipping 106 matching lines @@
       "--perf-id=ClangToTMacASan",
       "--perf-config={'r_clang_rev': '123456'}",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/chromium/sizes.py",
       "--target",
       "Release",
       "--platform",
       "mac"
     ],
     "cwd": "[BUILDER_CACHE]/ClangToTMacASan",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "LLVM_FORCE_HEAD_REVISION": "YES"
     },
     "name": "sizes"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]