Add presubmit check to warn against using NULL

PRESUBMIT.py

Index: PRESUBMIT.py
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 16f00ad5bff741fd78119cda7f62031447597c1d..3ba67dd1169d02c9425ae09384a272ea2512b4bc 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -155,6 +155,14 @@ _BANNED_CPP_FUNCTIONS = (
     # 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',