Add PRESUBMIT to disallow new usage of FOR_EACH_OBSERVER.

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 289 matching lines @@
       ),
     ),
     (
       '#pragma comment(lib,',
       (
         'Specify libraries to link with in build files and not in the source.',
       ),
       True,
       (),
     ),
+    (
+      r'FOR_EACH_OBSERVER(',
+      (
+        'Use range-based for loops to iterate through base::ObserverList, e.g.',
+        '        for (auto& observer : observers) { observer.Observe(); }',
+      ),
+      True,
+      (),
+    ),
 )
 
 
 _IPC_ENUM_TRAITS_DEPRECATED = (
     'You are using IPC_ENUM_TRAITS() in your code. It has been deprecated.\n'
     'See http://www.chromium.org/Home/chromium-security/education/security-tips-for-ipc')
 
 
 _VALID_OS_MACROS = (
     # Please keep sorted.
@@ skipping 1944 matching lines @@
   results.extend(input_api.canned_checks.CheckTreeIsOpen(
       input_api,
       output_api,
       json_url='http://chromium-status.appspot.com/current?format=json'))
 
   results.extend(input_api.canned_checks.CheckChangeHasBugField(
       input_api, output_api))
   results.extend(input_api.canned_checks.CheckChangeHasDescription(
       input_api, output_api))
   return results