Turn on hermetic toolchain for the FORCE_MAC_TOOLCHAIN fyi bot.

tools/mb/mb_config.pyl

Index: tools/mb/mb_config.pyl
diff --git a/tools/mb/mb_config.pyl b/tools/mb/mb_config.pyl
index 05304c4fdcfabcbf0a035cc97e490a51eadb7599..d1b35c816a1c79d6bb76500c243ba4ea7397f7d4 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -99,7 +99,7 @@
       'Chromium Linux32 Goma Canary (clobber)': 'release_bot_x86',
       'Chromium Mac 10.10 MacViews': 'mac_views_browser_release_bot',
       'Chromium Mac 10.11': 'release_bot',
-      'Chromium Mac 10.11 Force Mac Toolchain': 'release_bot',
+      'Chromium Mac 10.11 Force Mac Toolchain': 'release_bot_mac_hermetic',
       'Chromium Mac 10.9 Goma Canary': 'release_bot',
       'Chromium Mac 10.9 Goma Canary (clobber)': 'release_bot',
       'Chromium Mac 10.9 Goma Canary (dbg)': 'debug_bot',
@@ -471,7 +471,7 @@
       'android_compile_x86_dbg': 'android_debug_trybot_x86',
       'android_coverage': 'android_debug_trybot_java_coverage',
       'android_cronet': 'android_cronet_release_trybot_arm_no_neon',
- 
+
       # TODO(crbug/597596): Switch this back to debug_trybot when cronet's
       # shared library loading is fixed.
       'android_cronet_tester': 'android_cronet_debug_static_bot_arm_no_neon',
@@ -1329,6 +1329,10 @@
       'release_bot', 'chrome_with_codecs',
     ],
 
+    'release_bot_mac_hermetic': [
+      'release_bot', 'mac_hermetic_toolchain',
+    ],
+
     'release_bot_mac_strip': [
       'release_bot', 'mac_strip',
     ],
@@ -1633,6 +1637,10 @@
       'mixins': ['chrome_with_codecs'],
     },
 
+    'mac_hermetic_toolchain': {
+      'gn_args': 'use_system_xcode=false',
+    },
+
     'hybrid': {
       'gn_args': 'v8_target_cpu="arm" target_cpu="x86"',
       'mixins': ['disable_nacl'],