Add canned presubmit check for GN formatting

presubmit_canned_checks.py

 # 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.
 
 """Generic presubmit checks that can be reused by other presubmit checks."""
 
 import os as _os
 _HERE = _os.path.dirname(_os.path.abspath(__file__))
 
 
@@ skipping 1095 matching lines @@
   code, _ = git_cl.RunGitWithCode(cmd, suppress_stderr=True)
   if code == 2:
     return [output_api.PresubmitPromptWarning(
       'The %s directory requires clang-formatting. '
       'Please run git cl format %s' %
       (input_api.basename(input_api.PresubmitLocalPath()),
        input_api.basename(input_api.PresubmitLocalPath())))]
   # As this is just a warning, ignore all other errors if the user
   # happens to have a broken clang-format, doesn't use git, etc etc.
   return []
+
+
+def CheckGNFormatted(input_api, output_api):
+  import gn
+  affected_files = input_api.AffectedFiles(
+      include_deletes=False,
+      file_filter=lambda x: x.LocalPath().endswith('.gn') or
+                            x.LocalPath().endswith('.gni'))
+  warnings = []
+  for f in affected_files:
+    cmd = ['gn', 'format', '--dry-run', f.AbsoluteLocalPath()]
+    rc = gn.main(cmd)
+    if rc == 2:
+      warnings.append(output_api.PresubmitPromptWarning(
+          '%s requires formatting. Please run `gn format --in-place %s`.' % (
+              f.AbsoluteLocalPath(), f.LocalPath())))
+  # It's just a warning, so ignore other types of failures assuming they'll be
+  # caught elsewhere.
+  return warnings