[infra] Add mirrors WATCHLIST.

R=zra@google.com

Review-Url: https://codereview.chromium.org/2995633002 .

WATCHLISTS

@@ -35,6 +35,9 @@
         ')$'
       )
     },
+    'mirrors' : {
+      'filepath': '.*mirrors.*',
+    },
     'observatory': {
       'filepath': (
         '^runtime/bin/vmservice/|'
@@ -53,6 +56,7 @@
     'build': [ 'zra@google.com' ],
     'front_end': [ 'dart-fe-team+reviews@google.com' ],
     'messages_review': [ 'dart-uxr+reviews@google.com' ],
+    'mirrors' : [ 'rmacnak@google.com' ],
     'observatory': [ 'turnidge@google.com', 'rmacnak@google.com' ],
     'runtime': [ 'vm-dev@dartlang.org' ],
   },