Add presubmit check to warn against using NULL

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 depot_tools.
 """
 
@@ skipping 137 matching lines @@
       True,
     ),
 )
 
 
 _BANNED_CPP_FUNCTIONS = (
     # Make sure that gtest's FRIEND_TEST() macro is not used; the
     # FRIEND_TEST_ALL_PREFIXES() macro from base/gtest_prod_util.h should be
     # used instead since that allows for FLAKY_ and DISABLED_ prefixes.
     (
+      r'\bNULL\b',
+      (
+       'New code should not use NULL. Use nullptr instead.',
+      ),
+      True,
+      (),
+    ),
+    (
       'FRIEND_TEST(',
       (
        'Chromium code should not use gtest\'s FRIEND_TEST() macro. Include',
        'base/gtest_prod_util.h and use FRIEND_TEST_ALL_PREFIXES() instead.',
       ),
       False,
       (),
     ),
     (
       r'XSelectInput|CWEventMask|XCB_CW_EVENT_MASK',
@@ skipping 2204 matching lines @@
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(
       input_api.canned_checks.CheckPatchFormatted(input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results