Remove reference to old gn win x64 bot in roll_gn.py.

tools/gn/bin/roll_gn.py

 #!/usr/bin/env python
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """An auto-roller for GN binaries into Chromium.
 
 This script is used to update the GN binaries that a Chromium
 checkout uses. In order to update the binaries, one must follow
 four steps in order:
@@ skipping 414 matching lines @@
     return (
       'Roll buildtools %s..%s\n'
       '\n'
       '  In order to roll GN %s..%s (r%s:r%s) and pick up\n'
       '  the following changes:\n'
       '\n'
       '%s'
       '\n'
       'TBR=%s\n'
       'CQ_EXTRA_TRYBOTS=tryserver.chromium.mac:mac_chromium_gn_dbg;'
-      'tryserver.chromium.win:win8_chromium_gn_dbg,'
-      'win_chromium_gn_x64_rel\n' % (
+      'tryserver.chromium.win:win8_chromium_gn_dbg\n' % (
         old_buildtools_commitish[:COMMITISH_DIGITS],
         new_buildtools_commitish[:COMMITISH_DIGITS],
         self.old_gn_commitish[:COMMITISH_DIGITS],
         self.new_gn_commitish[:COMMITISH_DIGITS],
         self.old_gn_version,
         self.new_gn_version,
         gn_changes,
         self.reviewer,
       ))
 
   def GetGNChanges(self):
     _, out, _ = self.Call(
         "git log --pretty='  %h %s' " +
         "%s..%s tools/gn" % (self.old_gn_commitish, self.new_gn_commitish))
     return out
 
   def Call(self, cmd, cwd=None):
     proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, shell=True,
                             cwd=(cwd or self.chromium_src_dir))
     out, err = proc.communicate()
     return proc.returncode, out, err
 
 
 if __name__ == '__main__':
   roller = GNRoller()
   sys.exit(roller.Roll())