Update default trybots.

PRESUBMIT.py

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 226 matching lines @@
         json_url='http://v8-status.appspot.com/current?format=json'))
   return results
 
 
 def GetPreferredTryMasters(project, change):
   return {
     'tryserver.v8': {
       'v8_linux_rel': set(['defaulttests']),
       'v8_linux_dbg': set(['defaulttests']),
       'v8_linux_nosnap_rel': set(['defaulttests']),
-      'v8_linux_nosnap_dbg': set(['defaulttests']),
       'v8_linux64_rel': set(['defaulttests']),
       'v8_linux_arm_dbg': set(['defaulttests']),
       'v8_linux_arm64_rel': set(['defaulttests']),
       'v8_linux_layout_dbg': set(['defaulttests']),
+      'v8_linux_chromium_gn_rel': set(['defaulttests']),
       'v8_mac_rel': set(['defaulttests']),
       'v8_win_rel': set(['defaulttests']),
       'v8_win64_compile_rel': set(['defaulttests']),
     },
   }