Enable MSVC warning C4018 for various directories that seem to pass as-is.

third_party/usrsctp/usrsctp.gyp

 # 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.
 {
   'variables': {
     'libsctp_target_type%': 'static_library',
   },
   'target_defaults': {
     'defines': [
       'SCTP_PROCESS_LEVEL_LOCKS',
@@ skipping 144 matching lines @@
           ],
           'defines!': [
             # Remove Chrome's WINVER defines to avoid redefinition warnings.
             'WINVER=0x0603',
             '_WIN32_WINNT=0x0603',
           ],
           'cflags!': [ '/W3', '/WX' ],
           'cflags': [ '/w' ],
           # TODO(ldixon) : Remove this disabling of warnings by pushing a
           # fix upstream to usrsctp
-          'msvs_disabled_warnings': [ 4002, 4013, 4018, 4133, 4267, 4313, 4700 ],
+          'msvs_disabled_warnings': [ 4002, 4013, 4133, 4267, 4313, 4700 ],
         }, {  # OS != "win",
           'defines': [
             'NON_WINDOWS_DEFINE',
           ],
         }],
       ],  # conditions
     },  # target usrsctp
   ],  # targets
 }