Fix and enable MSVC warning C4018 for base/.

base/base.gypi

 # 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.
 
 {
   'includes': [
     'trace_event/trace_event.gypi',
   ],
   'target_defaults': {
     'variables': {
@@ skipping 746 matching lines @@
           'win/wrapped_window_proc.cc',
           'win/wrapped_window_proc.h',
           '<@(trace_event_sources)',
         ],
         'defines': [
           'BASE_IMPLEMENTATION',
         ],
         'include_dirs': [
           '..',
         ],
-        'msvs_disabled_warnings': [
-          4018,
-        ],
         'target_conditions': [
           ['OS == "mac" or OS == "ios"', {
             'sources!': [
               'memory/shared_memory_posix.cc',
             ],
           }],
           ['(<(desktop_linux) == 0 and <(chromeos) == 0) or >(nacl_untrusted_build)==1', {
               'sources/': [
                 ['exclude', '^nix/'],
               ],
@@ skipping 257 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }