Add ChangeLog presubmit check.

PRESUBMIT.py

 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 51 matching lines @@
   """Check the env var whether we want to skip tree check.
      Only skip if src/version.cc has been updated."""
   src_version = 'src/version.cc'
   FilterFile = lambda file: file.LocalPath() == src_version
   if not input_api.AffectedSourceFiles(
       lambda file: file.LocalPath() == src_version):
     return False
   return input_api.environ.get('PRESUBMIT_TREE_CHECK') == 'skip'
 
 
+def _CheckChangeLogFlag(input_api, output_api):
+  """Checks usage of LOG= flag in the commit message."""
+  results = []
+  if input_api.change.BUG and not 'LOG' in input_api.change.tags:
+    results.append(output_api.PresubmitError(
+        'An issue reference (BUG=) requires a change log flag (LOG=). '
+        'Use LOG=Y for including this commit message in the change log. '
+        'Use LOG=N or leave blank otherwise.'))
+  return results
+
+
 def CheckChangeOnUpload(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
+  results.extend(_CheckChangeLogFlag(input_api, output_api))
   return results
 
 
 def CheckChangeOnCommit(input_api, output_api):
   results = []
   results.extend(_CommonChecks(input_api, output_api))
+  results.extend(_CheckChangeLogFlag(input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   if not _SkipTreeCheck(input_api, output_api):
     results.extend(input_api.canned_checks.CheckTreeIsOpen(
         input_api, output_api,
         json_url='http://v8-status.appspot.com/current?format=json'))
   return results