Set landmines on x64 too.

build/landmines.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """
 This file holds a list of reasons why a particular build needs to be clobbered
 (or a list of 'landmines').
 
 This script runs every build as a hook. If it detects that the build should
@@ skipping 205 matching lines @@
   options, args = parser.parse_args()
 
   if args:
     parser.error('Unknown arguments %s' % args)
 
   logging.basicConfig(
       level=logging.DEBUG if options.verbose else logging.ERROR)
 
   gyp_helper.apply_chromium_gyp_env()
 
-  for target in ('Debug', 'Release'):
+  for target in ('Debug', 'Release', 'Debug_x64', 'Release_x64'):
     set_up_landmines(target)
 
   return 0
 
 
 if __name__ == '__main__':
   sys.exit(main())