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

chrome/chrome_common.gypi

 # Copyright (c) 2013 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.
 
 {
   'targets': [
     {
       'target_name': 'common',
       'type': 'static_library',
       'variables': {
@@ skipping 518 matching lines @@
         ['enable_automation==0', {
           'sources/': [
             ['exclude', '^common/automation_']
           ]
         }],
         ['enable_plugins==0', {
           'source!' : [
             'common/pepper_permission_util.cc',
           ],
         }],
-        ['use_system_nspr==1', {
-          'dependencies': [
-            '<(DEPTH)/base/third_party/nspr/nspr.gyp:nspr',
-          ],
-        }],
         ['enable_webrtc==0', {
           'sources!': [
             'common/media/webrtc_logging_messages.h',
           ]
         }],
         ['enable_printing==0', {
           'sources!': [
             'common/print_messages.cc',
             'common/print_messages.h',
           ]
@@ skipping 189 matching lines @@
         'common/metrics/proto/user_action_event.proto',
       ],
       'variables': {
         'proto_in_dir': 'common/metrics/proto',
         'proto_out_dir': 'chrome/common/metrics/proto',
       },
       'includes': [ '../build/protoc.gypi' ],
     },
   ],
 }