Make NSS work in GN build on Windows.

build/config/win/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/win/visual_studio_version.gni")
 
 # Compiler setup for the Windows SDK. Applied to all targets.
 config("sdk") {
   # The include path is the stuff returned by the script.
   #include_dirs = msvc_config[0]  TODO(brettw) make this work.
 
   defines = [
     "_ATL_NO_OPENGL",
     "_SECURE_ATL",
     "_WIN32_WINNT=0x0602",
     "_WINDOWS",
     "CERT_CHAIN_PARA_HAS_EXTRA_FIELDS",
     "NOMINMAX",
     "NTDDI_VERSION=0x06020000",
     "PSAPI_VERSION=1",
     "WIN32",
-    "WIN32_LEAN_AND_MEAN",
     "WINVER=0x0602",
   ]
 
   # The Windows SDK include directories must be first. They both have a sal.h,
   # and the SDK one is newer and the SDK uses some newer features from it not
   # present in the Visual Studio one.
   include_dirs = [
     "$windows_sdk_path\Include\shared",
     "$windows_sdk_path\Include\um",
     "$windows_sdk_path\Include\winrt",
@@ skipping 69 matching lines @@
 }
 
 # Incremental linking ----------------------------------------------------------
 
 config("incremental_linking") {
   ldflags = [ "/INCREMENTAL" ]
 }
 config("no_incremental_linking") {
   ldflags = [ "/INCREMENTAL:NO" ]
 }
+
+# Character set ----------------------------------------------------------------
+
+# Not including this config means "ansi" (8-bit system codepage).
+config("unicode") {
+  defines = [
+    "_UNICODE",
+    "UNICODE",
+  ]
+}
+
+# Lean and mean ----------------------------------------------------------------
+
+# Some third party code might not compile with WIN32_LEAN_AND_MEAN so we have
+# to have a separate config for it. Remove this config from your target to
+# get the "bloaty and accomodating" version of windows.h.
+config("lean_and_mean") {
+  defines = [
+    "WIN32_LEAN_AND_MEAN",
+  ]
+}