Build NaCl browser tests with GN

ppapi/native_client/src/untrusted/irt_stub/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.
 
 assert(is_nacl,
        "These targets must only be built using the untrusted NaCl toolchains.")
 
-source_set("ppapi_stub_lib") {
+static_library("ppapi_stub_lib") {
+  # This library is distributed as a part of the SDK and as such has to
+  # be a static library rather than a source set.
   sources = [
     "plugin_main_irt.c",
     "ppapi_plugin_main.c",
     "ppapi_plugin_start.c",
     "thread_creator.c",
   ]
   cflags = [ "-fPIC" ]
   deps = [
     "//build/config/nacl:nacl_base",
   ]
 }