Adds a presubmit that looks for version control conflicts.

PRESUBMIT.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.
 
 """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 617 matching lines @@
       warnings.extend(_CheckIncludeOrderInFile(input_api, f, False,
                                                changed_linenums))
 
   results = []
   if warnings:
     results.append(output_api.PresubmitPromptWarning(_INCLUDE_ORDER_WARNING,
                                                      warnings))
   return results
 
 
+def _CheckForVersionControlConflictsInFile(input_api, f):
+  pattern = input_api.re.compile('^(?:<<<<<<<|>>>>>>>) |^=======$')
+  errors = []
+  for line_num, line in f.ChangedContents():
+    if pattern.match(line):
+      errors.append('    %s:%d %s' % (f.LocalPath(), line_num, line))
+  return errors
+
+
+def _CheckForVersionControlConflicts(input_api, output_api):
+  """Usually this is not intentional and will cause a compile failure."""
+  errors = []
+  for f in input_api.AffectedFiles():
+    errors.extend(_CheckForVersionControlConflictsInFile(input_api, f))
+
+  results = []
+  if errors:
+    results.append(output_api.PresubmitError(
+      'Version control conflict markers found, please resolve.', errors))
+  return results
+
+
 def _CommonChecks(input_api, output_api):
   """Checks common to both upload and commit."""
   results = []
   results.extend(input_api.canned_checks.PanProjectChecks(
       input_api, output_api, excluded_paths=_EXCLUDED_PATHS))
   results.extend(_CheckAuthorizedAuthor(input_api, output_api))
   results.extend(
     _CheckNoProductionCodeUsingTestOnlyFunctions(input_api, output_api))
   results.extend(_CheckNoIOStreamInHeaders(input_api, output_api))
   results.extend(_CheckNoUNIT_TESTInSourceFiles(input_api, output_api))
   results.extend(_CheckNoNewWStrings(input_api, output_api))
   results.extend(_CheckNoDEPSGIT(input_api, output_api))
   results.extend(_CheckNoBannedFunctions(input_api, output_api))
   results.extend(_CheckNoPragmaOnce(input_api, output_api))
   results.extend(_CheckNoTrinaryTrueFalse(input_api, output_api))
   results.extend(_CheckUnwantedDependencies(input_api, output_api))
   results.extend(_CheckFilePermissions(input_api, output_api))
   results.extend(_CheckNoAuraWindowPropertyHInHeaders(input_api, output_api))
   results.extend(_CheckIncludeOrder(input_api, output_api))
+  results.extend(_CheckForVersionControlConflicts(input_api, output_api))
 
   if any('PRESUBMIT.py' == f.LocalPath() for f in input_api.AffectedFiles()):
     results.extend(input_api.canned_checks.RunUnitTestsInDirectory(
         input_api, output_api,
         input_api.PresubmitLocalPath(),
         whitelist=[r'.+_test\.py$']))
   return results
 
 
 def _CheckSubversionConfig(input_api, output_api):
@@ skipping 133 matching lines @@
 
   # Match things like path/aura/file.cc and path/file_aura.cc.
   # Same for ash and chromeos.
   if any(re.search('[/_](ash|aura)', f) for f in files):
     trybots += ['linux_chromeos_clang:compile', 'win_aura',
                 'linux_chromeos_asan']
   elif any(re.search('[/_]chromeos', f) for f in files):
     trybots += ['linux_chromeos_clang:compile', 'linux_chromeos_asan']
 
   return trybots