Linux: add option to use system nspr.

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,
     },
     'target_conditions': [
       # This part is shared between the targets defined below.
       ['base_target==1', {
         'sources': [
           '../build/build_config.h',
           'third_party/dmg_fp/dmg_fp.h',
           'third_party/dmg_fp/g_fmt.cc',
           'third_party/dmg_fp/dtoa_wrapper.cc',
           'third_party/icu/icu_utf.cc',
           'third_party/icu/icu_utf.h',
-          'third_party/nspr/prtime.cc',
-          'third_party/nspr/prtime.h',
-          'third_party/nspr/prcpucfg_linux.h',
           'third_party/xdg_mime/xdgmime.h',
           'allocator/allocator_extension.cc',
           'allocator/allocator_extension.h',
           'allocator/type_profiler_control.cc',
           'allocator/type_profiler_control.h',
           'android/base_jni_registrar.cc',
           'android/base_jni_registrar.h',
           'android/build_info.cc',
           'android/build_info.h',
           'android/scoped_java_ref.cc',
@@ skipping 689 matching lines @@
           # 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$'] ],
           }],
         ],
       }],
     ],
   },
 }