Add a PRESUBMIT check for header files that are missing the |#pragma once| directive.

PRESUBMIT.py

 # Copyright (c) 2010 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.
 
 """Top-level presubmit script for Chromium.
 
 See http://dev.chromium.org/developers/how-tos/depottools/presubmit-scripts
 for more details about the presubmit API built into gcl.
 """
 
@@ skipping 42 matching lines @@
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeSvnEolStyle(
       input_api, output_api, source_file_filter=text_files))
   results.extend(input_api.canned_checks.CheckSvnForCommonMimeTypes(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckLicense(
       input_api, output_api, _LICENSE_HEADER, source_file_filter=sources))
   return results
 
 
+def _CheckPragmaOnce(input_api, output_api):
+  """Checks to make sure all new headers have |#pragma once|."""
+  files = []
+  for f in input_api.change.AffectedTextFiles():
+    if f.LocalPath().endswith('.h'):
+      contents = input_api.ReadFile(f)
+      if not '#pragma once' in contents.splitlines(False):
+        files.append(f)
+  
+  if len(files):
+    if input_api.is_committing:
+      res_type = output_api.PresubmitPromptWarning
+    else:
+      res_type = output_api.PresubmitNotifyResult
+    return [ res_type('Missing |#pragma once| directive:', files) ]
+  return []
+
+
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
+  results.extend(_CheckPragmaOnce(input_api, output_api))
   return results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   results = []
   if not input_api.json:
     results.append(output_api.PresubmitNotifyResult(
         'You don\'t have json nor simplejson installed.\n'
         '  This is a warning that you will need to upgrade your python '
         'installation.\n'
@@ skipping 29 matching lines @@
       input_api,
       output_api,
       'http://build.chromium.org/buildbot/waterfall/json/builders?filter=1',
       6,
       IGNORED_BUILDERS))
   return results
 
 
 def GetPreferredTrySlaves():
   return ['win', 'linux', 'mac']