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

third_party/leveldatabase/BUILD.gn

 # Copyright 2014 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.
 
 import("//testing/test.gni")
 
 defines = [ "LEVELDB_PLATFORM_CHROMIUM=1" ]
 
 config("leveldatabase_config") {
   include_dirs = [
@@ skipping 96 matching lines @@
   configs += [ "//build/config/compiler:no_chromium_code" ]
 
   public_configs = [ ":leveldatabase_config" ]
 
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
     "//third_party/re2",
     "//third_party/snappy",
   ]
+
+  if (is_win) {
+    cflags = [ "/wd4018" ]  # Signed/unsigned mismatch in comparison.
+  }
 }
 
 if (!is_android) {
   test("env_chromium_unittests") {
     sources = [
       "env_chromium_unittest.cc",
     ]
     deps = [
       ":leveldatabase",
       "//base/test:test_support",
@@ skipping 191 matching lines @@
     sources = [
       "src/db/write_batch_test.cc",
     ]
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
     deps = [
       ":leveldb_testutil",
     ]
   }
 }