Remove use_system_nspr build flag. It's not "officially" supported

base/base.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.
 
 {
   'target_defaults': {
     'variables': {
       'base_target': 0,
       'base_i18n_target': 0,
     },
@@ skipping 912 matching lines @@
               ['exclude', '^file_util_linux\\.cc$'],
               ['exclude', '^process/process_linux\\.cc$'],
               ['exclude', '^sys_info_linux\\.cc$'],
             ],
           }],
           # Remove all unnecessary files for build_nexe.py to avoid exceeding
           # command-line-string limitation when building NaCl on Windows.
           ['OS == "win" and >(nacl_untrusted_build)==1', {
               'sources/': [ ['exclude', '\\.h$'] ],
           }],
-          ['<(use_system_nspr)==1 and >(nacl_untrusted_build)==0', {
-            'sources/': [
-              ['exclude', '^third_party/nspr/'],
-            ],
-          }],
           ['<(toolkit_uses_gtk) == 1', {
             'sources!': [
               'x11/x11_error_tracker.cc',
             ],
           }],
         ],
       }],
       ['base_i18n_target==1', {
         'defines': [
           'BASE_I18N_IMPLEMENTATION',
@@ skipping 32 matching lines @@
           'i18n/time_formatting.h',
           'i18n/timezone.cc',
           'i18n/timezone.h',
           'i18n/utf8_validator_tables.cc',
           'i18n/utf8_validator_tables.h',
         ],
       }]
     ],
   },
 }