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

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_swarming.json

 [
   {
     "cmd": [
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/ios/host_info.py",
       "--json-file",
       "/path/to/tmp/json"
     ],
     "name": "host and tools info",
     "~followup_annotations": [
       "@@@STEP_TEXT@<br />OS X 1.2.3, Xcode 6.7.8 (5D342509a)@@@",
@@ skipping 180 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/mac_swarming/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json",
       "GYP_DEFINES": "clang=1 component=shared_library dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[CACHE]/cipd/goma' rtc_use_h264=1 target_arch=x64 test_isolation_mode=prepare use_goma=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
@@ skipping 20 matching lines @@
       "[BUILDER_CACHE]/mac_swarming/src/webrtc/build/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--swarming-targets-file",
       "audio_decoder_unittests\ncommon_audio_unittests\ncommon_video_unittests\nmodules_tests\nmodules_unittests\npeerconnection_unittests\nrtc_media_unittests\nrtc_pc_unittests\nrtc_stats_unittests\nrtc_unittests\nsystem_wrappers_unittests\ntest_support_unittests\ntools_unittests\nvideo_engine_tests\nvoice_engine_unittests\nwebrtc_nonparallel_tests\nxmllite_xmpp_unittests\n",
       "--gyp-script=[BUILDER_CACHE]/mac_swarming/src/webrtc/build/gyp_webrtc.py",
       "//out/Debug"
     ],
     "cwd": "[BUILDER_CACHE]/mac_swarming/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@audio_decoder_unittests@@@",
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@common_audio_unittests@@@",
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@common_video_unittests@@@",
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@modules_tests@@@",
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@modules_unittests@@@",
       "@@@STEP_LOG_LINE@swarming-targets-file.txt@peerconnection_unittests@@@",
@@ skipping 16 matching lines @@
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[build]/scripts/slave/clang_revision.py",
       "--src-dir",
       "[BUILDER_CACHE]/mac_swarming/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 29 matching lines @@
       "/path/to/tmp/json",
       "--goma-service-account-json-file",
       "/creds/service_accounts/service-account-goma-client.json",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--goma-fail-fast",
       "--goma-disable-local-fallback",
       "--"
     ],
     "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 3119 matching lines @@
       "@@@STEP_LOG_END@json.output@@@",
       "@@@STEP_LINK@shard #0 isolated out@blah@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]