Flip Mac beta, stable configs to use GN.

I forgot to change these two entries before the M54 branch,
unfortunately, and this means that the M54 beta builder is
still trying to use GYP.

TBR=brettw@chromium.org
NOTRY=true
BUG=641634

Review-Url: https://codereview.chromium.org/2291903002
Cr-Commit-Position: refs/heads/master@{#415203}

tools/mb/mb_config.pyl

@@ -728,8 +728,8 @@
     },
 
     'official.desktop.continuous': {
-      'mac beta': 'gyp_official',
-      'mac stable': 'gyp_official',
+      'mac beta': 'gn_official',
+      'mac stable': 'gn_official',
       'mac trunk': 'gn_official',
       'precise64 beta': 'gn_official',
       'precise64 stable': 'gn_official',
@@ -1447,10 +1447,6 @@
       'gn', 'msan_no_origins', 'edge', 'release_bot',
     ],
 
-    'gyp_official': [
-      'gyp', 'official',
-    ],
-
     'gn_official_goma_chromeos': [
       'gn', 'official', 'goma', 'chromeos',
     ],