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

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

Index: scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Builder.json
diff --git a/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Builder.json b/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Builder.json
index 220dac7410282f1361df7321a57add442eaa0732..2650b7ee12e9f2cfa2c70d586d38a76a6450e8b7 100644
--- a/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Builder.json
+++ b/scripts/slave/recipes/chromium.expected/full_chromium_webrtc_fyi_Mac_Builder.json
@@ -198,6 +198,7 @@
     ],
     "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",
@@ -270,6 +271,7 @@
     ],
     "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"
@@ -286,6 +288,7 @@
       "/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",
@@ -340,6 +343,7 @@
       "remoting_unittests"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",