Update "git cl format" for GN changes.

git_cl.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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld and Gerrit."""
 
 from __future__ import print_function
@@ skipping 4965 matching lines @@
       stdout = RunCommand(command, cwd=top_dir)
       if opts.dry_run and stdout:
         return_value = 2
     except dart_format.NotFoundError as e:
       print('Warning: Unable to check Dart code formatting. Dart SDK not '
             'found in this checkout. Files in other languages are still '
             'formatted.')
 
   # Format GN build files. Always run on full build files for canonical form.
   if gn_diff_files:
-    cmd = ['gn', 'format']
-    if not opts.dry_run and not opts.diff:
-      cmd.append('--in-place')
+    cmd = ['gn', 'format' ]
+    if opts.dry_run or opts.diff:
+      cmd.append('--dry-run')
     for gn_diff_file in gn_diff_files:
-      stdout = RunCommand(cmd + [gn_diff_file],
-                          shell=sys.platform == 'win32',
-                          cwd=top_dir)
-      if opts.diff:
-        sys.stdout.write(stdout)
+      gn_ret = subprocess2.call(cmd + [gn_diff_file],
+                                shell=sys.platform == 'win32',
+                                cwd=top_dir)
+      if opts.dry_run and gn_ret == 2:
+        return_value = 2  # Not formatted.
+      elif opts.diff and gn_ret == 2:
+        # TODO this should compute and print the actual diff.
+        print("This change has GN build file diff for " + gn_diff_file)
+      elif gn_ret != 0:
+        # For non-dry run cases (and non-2 return values for dry-run), a
+        # nonzero error code indicates a failure, probably because the file
+        # doesn't parse.
+        DieWithError("gn format failed on " + gn_diff_file +
+                     "\nTry running 'gn format' on this file manually.")
 
   return return_value
 
 
 @subcommand.usage('<codereview url or issue id>')
 def CMDcheckout(parser, args):
   """Checks out a branch associated with a given Rietveld or Gerrit issue."""
   _, args = parser.parse_args(args)
 
   if len(args) != 1:
@@ skipping 90 matching lines @@
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   setup_color.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)