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

chrome/test/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.
 
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//chrome/chrome_tests.gni")
 import("//chrome/test/base/js2gtest.gni")
@@ skipping 764 matching lines @@
           "nacl/pnacl_header_test.h",
         ]
 
         # TODO(GYP): Make NaCl work in other configs and update the irt
         # dependency as appropriate. We should probably push this
         # dependency into ppapi/native_client so that clients don't need
         # to know all of the different toolchain options.
         assert(current_cpu == "x64")
         data_deps += [
           # "test/data/nacl:shared_test_files",  # TODO(GYP) bug 512902
-          "//ppapi/native_client:nacl_irt(//native_client/build/toolchain/nacl:irt_x64)",
+          "//ppapi/native_client:nacl_irt(//build/toolchain/nacl:irt_x64)",
           # "../ppapi/ppapi_nacl.gyp:ppapi_nacl_tests",  # TODO(GYP) bug 512901
           # "../ppapi/tests/extensions/extensions.gyp:ppapi_tests_extensions_background_keepalive",  # TODO(GYP) bug 512900
           # "../ppapi/tests/extensions/extensions.gyp:ppapi_tests_extensions_media_galleries",  # TODO(GYP) bug 512900
           # "../ppapi/tests/extensions/extensions.gyp:ppapi_tests_extensions_packaged_app",  # TODO(GYP) bug 512900
         ]
         if (is_chromeos) {
           sources += [ "//third_party/liblouis/nacl_wrapper/liblouis_wrapper_browsertest.cc" ]
           deps += [
             "//chrome/browser/chromeos",
             #'../third_party/liblouis/liblouis_nacl.gyp:liblouis_test_data',  TODO(GYP) bug 512906
@@ skipping 1166 matching lines @@
     "//third_party/mojo/src/mojo/edk/system",
   ]
 
   if (!is_android && use_ash) {
     sources += rebase_path(
             chrome_tests_unit_gypi_values.chrome_test_support_unit_ash_sources,
             ".",
             "//chrome")
   }
 }