PPAPI/NaCl: Make a gyp flag for including the untrusted IPC proxy.

ppapi/ppapi_proxy_untrusted.gypi

 # Copyright (c) 2012 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.
 
 # This GYP file defines untrusted (NaCl) targets.  All targets in this
 # file should be conditionally depended upon via 'disable_nacl!=1' to avoid
 # requiring NaCl sources for building.
 
 {
   'includes': [
@@ skipping 394 matching lines @@
 #          '-lppruntime',
           '-pthread',
           '-lgpu_untrusted',
           '-lipc_untrusted',
           '-lbase_untrusted',
         ],
       },
     },
   ],
 }