Make NSS work in GN build on Windows.

build/config/gcc/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.
+
+# This config causes functions not to be automatically exported from shared
+# libraries. By default, all symbols are exported but this means there are
+# lots of exports that slow everything down. In general we explicitly mark
+# which functiosn we want to export from components.
+#
+# Some third_party code assumes all functions are exported so this is separated
+# into its own config so such libraries can remove this config to make symbols
+# public again.
+#
+# See http://gcc.gnu.org/wiki/Visibility
+config("symbol_visibility_hidden") {
+  # Note that -fvisibility-inlines-hidden is set globally in the compiler
+  # config since that can almost always be applied.
+  cflags = [ "-fvisibility=hidden" ]
+}