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

third_party/libxml/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.
 
 # Define an "os_include" variable that points at the OS-specific generated
 # headers.  These were generated by running the configure script offline.
 if (is_linux || is_android || is_nacl) {
   os_include = "linux"
 } else if (is_mac || is_ios) {
   os_include = "mac"
@@ skipping 137 matching lines @@
 
   public_configs = [ ":libxml_config" ]
   public_deps = [
     "//third_party/icu:icuuc",
   ]
   deps = [
     "//third_party/zlib",
   ]
 
   if (is_win) {
-    cflags_c = [ "/wd4101" ]  # Unreferenced local variable.
+    cflags_c = [
+      "/wd4018",  # Signed/unsigned mismatch in comparison.
+      "/wd4101",  # Unreferenced local variable.
+    ]
   } else if (is_mac || is_ios || is_android) {
     # http://www.xmlsoft.org/threads.html says that this is required when using
     # libxml from several threads, which can possibly happen in chrome. On
     # linux, this is picked up by transitivity from pkg-config output from
     # build/linux/system.gyp.
     defines = [ "_REENTRANT" ]
   }
 
   if (is_clang) {
     cflags = [
@@ skipping 22 matching lines @@
         # libxml passes a volatile LPCRITICAL_SECTION* to a function expecting
         # a void* volatile*.
         "-Wno-incompatible-pointer-types",
       ]
     }
   }
   configs += [ ":libxml_warnings" ]
 
   include_dirs = [ "$os_include" ]
 }