Actually flip the chromium.mac Mac builders to GN after r400895.

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 8acaeaca23381ba8fe52ac2727436267ec9480fd..41c6cb878f40cbfe612ee7fff4ab1c1e98a77a9d 100644
--- a/tools/mb/mb_config.pyl
+++ b/tools/mb/mb_config.pyl
@@ -347,8 +347,8 @@
     },
 
     'chromium.mac': {
-      'Mac Builder': 'swarming_gpu_tests_gyp_release_bot',
-      'Mac Builder (dbg)': 'swarming_gyp_debug_bot',
+      'Mac Builder': 'swarming_gpu_tests_gn_release_bot',
+      'Mac Builder (dbg)': 'swarming_gn_debug_bot',
       'Mac GN (dbg)': 'gn_debug_bot',
       'Mac GN': 'gn_release_bot',
       'Mac10.10 Tests': 'none',
@@ -1680,6 +1680,10 @@
       'swarming', 'gpu_tests', 'gn', 'release_trybot', 'x86',
     ],
 
+    'swarming_gpu_tests_gn_release_bot': [
+      'swarming', 'gpu_tests', 'gn', 'release_bot',
+    ],
+
     'swarming_gpu_tests_gyp_release_bot': [
       'swarming', 'gpu_tests', 'gyp', 'release_bot',
     ],
@@ -1830,10 +1834,6 @@
       'release', 'x86',
     ],
 
-    'swarming_gyp_debug_bot': [
-      'swarming', 'gyp', 'debug_bot',
-    ],
-
     'swarming_gn_release_trybot_arm': [
       'swarming', 'gn', 'release_trybot', 'arm', 'crosscompile',
     ],