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

third_party/re2/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.
 
 config("re2_config") {
   include_dirs = [ "." ]
 }
 
 static_library("re2") {
   sources = [
@@ skipping 49 matching lines @@
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   public_configs = [ ":re2_config" ]
 
   deps = [
     "//base/third_party/dynamic_annotations",
   ]
 
   if (is_win) {
     include_dirs = [ "mswin" ]
-    cflags = [ "/wd4722" ]  # Destructor never terminates.
+    cflags = [
+      "/wd4018",  # Signed/unsigned mismatch in comparison.
+      "/wd4722",  # Destructor never terminates.
+    ]
   } else {
     sources -= [ "mswin/stdint.h" ]
   }
 }