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

build/config/compiler/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.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
 }
 if (current_cpu == "mipsel" || current_cpu == "mips64el") {
@@ skipping 705 matching lines @@
 default_warning_flags_cc = []
 if (is_win) {
   # TODO(thakis): Make this unconditional once llvm.org/PR24167 is fixed.
   if (!is_clang || !is_asan) {
     default_warning_flags += [ "/WX" ]  # Treat warnings as errors.
   }
 
   default_warning_flags += [
     # Warnings permanently disabled:
 
+    # TODO(GYP) The GYP build doesn't have this globally enabled but disabled
+    # for a bunch of individual targets. Re-enable this globally when those
+    # targets are fixed.
+    "/wd4018",  # Comparing signed and unsigned values.
+
     # C4127: conditional expression is constant
     # This warning can in theory catch dead code and other problems, but
     # triggers in far too many desirable cases where the conditional
     # expression is either set by macros or corresponds some legitimate
     # compile-time constant expression (due to constant template args,
     # conditionals comparing the sizes of different types, etc.).  Some of
     # these can be worked around, but it's not worth it.
     "/wd4127",
 
     # C4251: 'identifier' : class 'type' needs to have dll-interface to be
@@ skipping 522 matching lines @@
       cflags += [ "-gsplit-dwarf" ]
     }
   }
 }
 
 config("no_symbols") {
   if (!is_win) {
     cflags = [ "-g0" ]
   }
 }