Fix GN build for gles2_conform_test on Windows.

gpu/gles2_conform_support/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.
 
 declare_args() {
   # Set to true to compile with the OpenGL ES 2.0 conformance tests.
   internal_gles2_conform_tests = false
 }
 
 config("gles2_conform_test_warnings") {
@@ skipping 124 matching lines @@
     if (is_linux) {
       if (!is_chromeos) {
         deps += [ "//build/config/linux/gtk2" ]
       }
     }
     if (is_win) {
       deps += [
         "//third_party/angle:libEGL",
         "//third_party/angle:libGLESv2",
       ]
-      defines = [
+      defines += [
         "EGLAPI=",
         "EGLAPIENTRY=",
       ]
-      defines -= [ "NOMINMAX" ]
+      configs -= [ "//build/config/win:nominmax" ]
       cflags = [
         "/wd4018",  # signed/unsigned mismatch
         "/wd4101",  # unreferenced local variable
         "/wd4715",  # not all control paths return a value
         "/wd4267",  # size_t/unsigned int conversion
       ]
     }
     if (is_mac) {
-      defines = [
+      defines += [
         "_STDINT",
         "_STDINT_H",
       ]
     }
 
     #'run_as': {
     #     'conditions': [
     #       ['OS=="win"', {
     #         'action': [
     #           '$(TargetPath)',
@@ skipping 26 matching lines @@
       ":gles2_conform_test_windowless",
     ]
     deps += [ ":generate_gles2_conform_tests" ]
     sources += [ "$target_gen_dir/gles2_conform_test_autogen.cc" ]
     data += [ "//third_party/gles2_conform/GTF_ES/" ]
 
     # TODO: Make these tests pull in the correct data dependencies once they
     # are exported in GN. Maybe from  //third_party/gles2_conform/GTF_ES/
   }
 }