[infra] Ensure args.gn is updated for the Linux cross builds during gclient runhooks.

tools/generate_buildfiles.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import argparse
 import os
 import subprocess
 import sys
 import utils
@@ skipping 28 matching lines @@
     '-m', 'all',
     '-a', 'arm,arm64',
     '--os', 'android',
   ]
   if options.verbose:
     gn_command.append('-v')
     print ' '.join(gn_command)
   return Execute(gn_command)
 
 
+def RunCrossGn(options):
+  if HOST_OS != 'linux':
+    return 0
+  gn_command = [
+    'python',
+    os.path.join(DART_ROOT, 'tools', 'gn.py'),
+    '-m', 'all',
+    '-a', 'arm,arm64',
+  ]
+  if options.verbose:
+    gn_command.append('-v')
+    print ' '.join(gn_command)
+  return Execute(gn_command)
+
+
 def RunHostGn(options):
   gn_command = [
     'python',
     os.path.join(DART_ROOT, 'tools', 'gn.py'),
     '-m', 'all',
     '-a', 'all',
   ]
   if options.verbose:
     gn_command.append('-v')
     print ' '.join(gn_command)
   return Execute(gn_command)
 
 
 def RunGn(options):
   status = RunHostGn(options)
   if status != 0:
     return status
+  status = RunCrossGn(options)
+  if status != 0:
+    return status
   return RunAndroidGn(options)
 
 
 def RunGyp(options):
   gyp_command = [
     'python',
     os.path.join(DART_ROOT, 'tools', 'gyp_dart.py'),
   ]
   if options.verbose:
     print ' '.join(gyp_command)
@@ skipping 31 matching lines @@
     return 0
   options = ParseArgs(argv)
   if options.gn:
     return RunGn(options)
   else:
     return RunGyp(options)
 
 
 if __name__ == '__main__':
     sys.exit(main(sys.argv))