Revert of Enable C4018 globally for the GN build, and disable per-target.

third_party/snappy/BUILD.gn

 # Copyright (c) 2013 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.
 
 config("snappy_config") {
   include_dirs = [ "src" ]
 
   # These OS-specific generated headers were made by running the configure
   # script offline.
   if (is_win) {
@@ skipping 13 matching lines @@
     "src/snappy-stubs-internal.cc",
     "src/snappy-stubs-internal.h",
     "src/snappy.cc",
     "src/snappy.h",
   ]
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   public_configs = [ ":snappy_config" ]
 
-  if (is_win) {
-    cflags = [ "/wd4018" ]  # Signed/unsigned mismatch in comparison.
-  }
-
   if (is_clang) {
     # snappy-stubs-internal.h unapologetically has: using namespace std
     # https://code.google.com/p/snappy/issues/detail?id=70
     configs -= [ "//build/config/clang:extra_warnings" ]
   }
 }