Update Chomium's build files to work w/ latest GN binaries.

ppapi/thunk/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.
 
 source_set("thunk") {
   sources = [
     "enter.cc",
     "enter.h",
     "ppb_audio_api.h",
     "ppb_audio_buffer_api.h",
@@ skipping 161 matching lines @@
       "ppb_video_decoder_dev_thunk.cc",
     ]
   }
 
   # We exclude a few more things for nacl_win64, to avoid pulling in more
   # dependencies.
   # TODO(GYP) this isn't correct. This should only be specifically for the
   # nacl_win64 build (cross-compiled for a 32-bit Chrome), rather than all
   # 64-bit builds.
   # See also //ppapi/shared_impl
-  if (is_win && cpu_arch == "x64" && current_toolchain != default_toolchain) {
+  if (is_win && current_cpu == "x64" && current_toolchain != default_toolchain) {
     sources += [
       "ppb_graphics_3d_thunk.cc",
       "ppb_host_resolver_private_thunk.cc",
       "ppb_tcp_server_socket_private_thunk.cc",
       "ppb_tcp_socket_private_thunk.cc",
       "ppb_udp_socket_private_thunk.cc",
       "ppb_x509_certificate_private_thunk.cc",
     ]
   }
 
   defines = [ "PPAPI_THUNK_IMPLEMENTATION" ]
 }