Make mock_nacl_gdb build on Win64

chrome/browser/nacl_host/test/mock_nacl_gdb.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.
 {
   'conditions': [
     ['disable_nacl==0 and (OS=="win" or OS=="linux")', {
       'targets': [
         {
           'target_name': 'mock_nacl_gdb',
           'type': 'executable',
           'include_dirs': [
             '../../../../',
           ],
           'sources': [
             'mock_nacl_gdb.cc',
           ],
           'dependencies': [
              '../../../../base/base.gyp:base',
           ],
+          # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
+          'msvs_disabled_warnings': [ 4267, ],
         },
       ],
     }],
   ],  
 }