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

third_party/protobuf/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.
 
 if (is_win) {
   config_h_dir = "vsprojects"
 } else {
   config_h_dir = "."
 }
 
@@ skipping 59 matching lines @@
   "src/google/protobuf/unknown_field_set.h",
   "src/google/protobuf/wire_format_lite.cc",
   "src/google/protobuf/wire_format_lite.h",
   "src/google/protobuf/wire_format_lite_inl.h",
   "$config_h_dir/config.h",
 ]
 
 protobuf_lite_cflags = []
 if (is_win) {
   protobuf_lite_cflags = [
-    "/wd4018",  # signed/unsigned mismatch in comparison
-    "/wd4244",  # implicit conversion, possible loss of data
-    "/wd4355",  # 'this' used in base member initializer list
-    "/wd4267",  # size_t to int truncation
-    "/wd4291",  # no matching operator delete for a placement new
+    "/wd4018",  # Signed/unsigned mismatch in comparison.
+    "/wd4244",  # Implicit conversion, possible loss of data.
+    "/wd4355",  # 'this' used in base member initializer list.
+    "/wd4267",  # Size_t to int truncation.
+    "/wd4291",  # No matching operator delete for a placement new.
   ]
 }
 
 component("protobuf_lite") {
   sources = protobuf_lite_sources
 
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
   if (is_win) {
     configs -= [ "//build/config/win:lean_and_mean" ]
@@ skipping 234 matching lines @@
     ":copy_google",
     ":copy_google_protobuf",
     ":copy_google_protobuf_internal",
   ]
 
   # Targets that depend on this should depend on the copied data files.
   data = get_target_outputs(":copy_google")
   data += get_target_outputs(":copy_google_protobuf")
   data += get_target_outputs(":copy_google_protobuf_internal")
 }