Fix GN build for gles2_conform_test on Windows.

gpu/gles2_conform_support/BUILD.gn

Index: gpu/gles2_conform_support/BUILD.gn
diff --git a/gpu/gles2_conform_support/BUILD.gn b/gpu/gles2_conform_support/BUILD.gn
index 5867daf552d3decfe41b240835f0621908073969..06b796f3f9870f0428c04a054c89eb9313f90f71 100644
--- a/gpu/gles2_conform_support/BUILD.gn
+++ b/gpu/gles2_conform_support/BUILD.gn
@@ -142,11 +142,11 @@ if (internal_gles2_conform_tests) {
         "//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
@@ -155,7 +155,7 @@ if (internal_gles2_conform_tests) {
       ]
     }
     if (is_mac) {
-      defines = [
+      defines += [
         "_STDINT",
         "_STDINT_H",
       ]