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

scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json

Index: scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json
diff --git a/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json b/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json
index ad340dadc63e13a76a6cd0d9e4fb40924d9d6646..b491d2dbde0eccb402993fb4e838a85091ec67ec 100644
--- a/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json
+++ b/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_Mac_Builder.json
@@ -195,6 +195,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Builder/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": "blink_logging_always_on=1 clang=1 component=static_library dcheck_always_on=1 ffmpeg_branding=Chrome gomadir='[CACHE]/cipd/goma' proprietary_codecs=1 target_arch=x64 use_goma=1",
@@ -267,6 +268,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/Mac_Builder/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
@@ -283,6 +285,7 @@
       "/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",
@@ -337,6 +340,7 @@
       "remoting_unittests"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",