Revert 187427 "PPAPI: Remove threading options; it's always on"

ppapi/ppapi_ipc_untrusted.gyp

 # 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.
 
 {
   'variables': {
     'chromium_code': 1,
   },
   'includes': [
     '../native_client/build/untrusted.gypi',
     'ppapi_ipc.gypi',
   ],
   'conditions': [
     ['disable_nacl==0 and disable_nacl_untrusted==0', {
       'targets': [
         {
           'target_name': 'ppapi_ipc_untrusted',
           'type': 'none',
           'variables': {
             'ppapi_ipc_target': 1,
             'nacl_win64_target': 0,
             'nacl_untrusted_build': 1,
             'nlib_target': 'libppapi_ipc_untrusted.a',
             'build_glibc': 0,
             'build_newlib': 1,
+            'defines': [
+              # Enable threading for the untrusted side of the proxy.
+              # TODO(bbudge) remove when this is the default.
+              'ENABLE_PEPPER_THREADING',
+            ],
           },
           'include_dirs': [
             '..',
           ],
           'dependencies': [
             '../native_client/tools.gyp:prep_toolchain',
             '../base/base_untrusted.gyp:base_untrusted',
             '../gpu/gpu_untrusted.gyp:gpu_ipc_untrusted',
             '../ipc/ipc_untrusted.gyp:ipc_untrusted',
             '../ppapi/ppapi_shared_untrusted.gyp:ppapi_shared_untrusted',
             '../components/tracing_untrusted.gyp:tracing_untrusted',
           ],
         },
       ],
     }],
   ],
 }