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

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 383819e2320ae3f8852a48b511a0d7f7cfc6d4eb..fd08ecb9b476e4d011c6bfa60e4f764d2a9b072c 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -60,9 +60,28 @@ def _CommonChecks(input_api, output_api):
   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