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

third_party/mesa/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.
 
 import("//build/config/ui.gni")
 
 config("mesa_headers_config") {
   include_dirs = [ "src/include" ]
   if (use_x11) {
     defines = [ "MESA_EGL_NO_X11_HEADERS" ]
@@ skipping 107 matching lines @@
 # mesa_internal_config is prepended to the config lists.  Flags that
 # disable warnings need to be appended instead so that they show up
 # after flags like -Wall. (gn orders flags on a target before flags from
 # configs.)
 config("mesa_internal_warnings") {
   cflags = []
   if (is_clang) {
     cflags += [
       "-Wno-tautological-constant-out-of-range-compare",
       "-Wno-mismatched-tags",  # Fixed upstream.
-
       # mesa's STATIC_ASSERT() macro expands to an ununused typedef.
       "-Wno-unused-local-typedef",
     ]
   }
   if (is_win) {
     # TODO(scottmg): http://crbug.com/143877 These should be removed if
     # Mesa is ever rolled and the warnings are fixed.
     cflags += [
       "/wd4005",  # Macro defined twice.
-      "/wd4018",  # Signed/unsigned mismatch in comparison.
       "/wd4065",  # Switch statement contains 'default' but no 'case' labels.
       "/wd4090",  # 'Operation' : different 'modifier' qualifiers
       "/wd4099",  # Type name struct-vs-class doesn't match.
       "/wd4273",  # Inconsistent DLL linkage.
       "/wd4291",  # No matching operator delete found for placement new.
       "/wd4305",  # Truncation from int to float.
       "/wd4334",  # Result of 32-bit shift implicitly converted to 64 bits.
       "/wd4345",  # POD-type default initializers.
     ]
   }
@@ skipping 569 matching lines @@
         "KEYWORD2=GLAPIENTRY",
       ]
     }
   }
 } else {
   #  Placeholder to allow targets to unconditionally depend on this.
   group("osmesa") {
   }
 }  # !is_android
 # TODO(GYP) Android osmesa_in_lib_dir target.