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

remoting/tools/javascript_key_tester/BUILD.gn

 # Copyright 2015 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/features.gni")
 
 if (enable_pnacl) {
   group("javascript_key_tester") {
     deps = [
       ":javascript_key_tester_copies",
     ]
   }
 
   copy("javascript_key_tester_copies") {
     pexe_dir = get_label_info(
-            "pnacl:remoting_key_tester_newlib(//native_client/build/toolchain/nacl:newlib_pnacl)",
+            "pnacl:remoting_key_tester_newlib(//build/toolchain/nacl:newlib_pnacl)",
             "root_out_dir")
 
     sources = [
       "$pexe_dir/remoting_key_tester_newlib.pexe",
       "background.js",
       "chord_tracker.js",
       "event_listeners.js",
       "icon_128.png",
       "main.css",
       "main.html",
       "main.js",
       "manifest.json",
       "pnacl/remoting_key_tester.nmf",
     ]
 
     outputs = [
       "$target_out_dir/remoting/key_tester/{{source_file_part}}",
     ]
 
     deps = [
       ":javascript_key_tester_jscompile",
-      "pnacl:remoting_key_tester_newlib(//native_client/build/toolchain/nacl:newlib_pnacl)",
+      "pnacl:remoting_key_tester_newlib(//build/toolchain/nacl:newlib_pnacl)",
     ]
   }
 
   group("javascript_key_tester_jscompile") {
     # TODO(GYP): crbug.com/471926 add support for run_jscompile=true.
     run_jscompile = false
     if (run_jscompile) {
     }
   }
 }