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

scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_gyp_rel.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 169 matching lines @@
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_PACKAGE_REPO[depot_tools]/gclient.py",
       "runhooks"
     ],
     "cwd": "[BUILDER_CACHE]/mac_gyp_rel/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 dcheck_always_on=1 fastbuild=1 ffmpeg_branding=Chrome gomadir='[CACHE]/cipd/goma' rtc_use_h264=1 target_arch=x64 use_goma=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILDER_CACHE]/mac_gyp_rel/src/tools/mb/mb.py",
       "gen",
       "-m",
       "tryserver.webrtc",
       "-b",
       "mac_gyp_rel",
       "--config-file",
       "[BUILDER_CACHE]/mac_gyp_rel/src/webrtc/build/mb_config.pyl",
       "--goma-dir",
       "[CACHE]/cipd/goma",
       "--gyp-script=[BUILDER_CACHE]/mac_gyp_rel/src/webrtc/build/gyp_webrtc.py",
       "//out/Release"
     ],
     "cwd": "[BUILDER_CACHE]/mac_gyp_rel/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]