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

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

Index: scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_compile_dbg.json
diff --git a/scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_compile_dbg.json b/scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_compile_dbg.json
index 05114fef0b37525ecc026aa4faf32c8fb692a075..ee211bb21ffca779282d939e3f8940b05b15c328 100644
--- a/scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_compile_dbg.json
+++ b/scripts/slave/recipes/webrtc/standalone.expected/tryserver_webrtc_mac_compile_dbg.json
@@ -187,6 +187,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/mac_compile_dbg/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 use_goma=1",
       "PATH": "%(PATH)s:RECIPE_PACKAGE_REPO[depot_tools]"
@@ -212,6 +213,7 @@
     ],
     "cwd": "[BUILDER_CACHE]/mac_compile_dbg/src",
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "generate_build_files"
@@ -228,6 +230,7 @@
       "/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",
@@ -277,6 +280,7 @@
       "--"
     ],
     "env": {
+      "FORCE_MAC_TOOLCHAIN": "1",
       "GOMA_SERVICE_ACCOUNT_JSON_FILE": "/creds/service_accounts/service-account-goma-client.json"
     },
     "name": "compile",