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

third_party/libexif/BUILD.gn

 # Copyright 2015 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.
 
 # Linux should use system libexif
 if (!is_linux || is_chromeos) {
   # TODO(thestig): Properly support building on Linux.
   config("libexif_config") {
     include_dirs = [ "sources" ]
     if (is_clang) {
@@ skipping 42 matching lines @@
 
     if (is_win) {
       defines = [
         # This seems like a hack, but this is what WebKit Win does.
         "snprintf=_snprintf",
         "inline=__inline",
       ]
 
       ldflags = [ "/DEF:" + rebase_path("libexif.def") ]
 
-      cflags = [ "/wd4018" ]  # size/unsigned mismatch
+      cflags = [ "/wd4018" ]  # Signed/unsigned mismatch in comparison.
 
       # TODO(GYP): Handle /analyze switch, when it will be used in GN.
     } else if (is_linux) {
       libs = [ "m" ]
     }
 
     configs -= [ "//build/config/compiler:chromium_code" ]
     configs += [ "//build/config/compiler:no_chromium_code" ]
 
     public_configs = [ ":libexif_config" ]
   }
 }