add nacl_toolchain to //build and flip to using it in the GN build.

build/config/nacl/BUILD.gn

Index: build/config/nacl/BUILD.gn
diff --git a/build/config/nacl/BUILD.gn b/build/config/nacl/BUILD.gn
new file mode 100644
index 0000000000000000000000000000000000000000..45421e21703ef03912b471fca621c95c7970474d
--- /dev/null
+++ b/build/config/nacl/BUILD.gn
@@ -0,0 +1,30 @@
+# Copyright (c) 2014 The Native Client Authors. All rights reserved.

[Dirk Pranke, 2015/08/28 02:29:41]

This file is simply copied over from //native_client/build/config/nacl/BUILD.gn

+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+import("nacl_defines.gni")
+
+# Native Client Definitions
+config("nacl_defines") {
+  defines = nacl_defines
+  include_dirs = [ "//." ]

[brettw, 2015/08/28 18:25:52]

This should come from //build/config/compiler:default_include_dirs which should
be applied by default to all targets. So you can remove it.

[Dirk Pranke, 2015/08/28 20:08:31]

Acknowledged.

+}
+
+config("nexe_defines") {
+  defines = [
+    "DYNAMIC_ANNOTATIONS_ENABLED=1",
+    "DYNAMIC_ANNOTATIONS_PREFIX=NACL_",
+  ]
+}
+
+# The base target that all targets in the NaCl build should depend on.
+# This allows configs to be modified for everything in the NaCl build, even when
+# the NaCl build is composed into the Chrome build.  (GN has no functionality to
+# add flags to everythin in //native_client, having a base target works around
+# that limitation.)
+source_set("nacl_base") {
+  public_configs = [ ":nacl_defines" ]
+  if (current_os == "nacl") {
+    public_configs += [ ":nexe_defines" ]
+  }
+}