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

build/common.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.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 974 matching lines @@
     'spdy_proxy_auth_value%': '<(spdy_proxy_auth_value)',
     'data_reduction_proxy_probe_url%': '<(data_reduction_proxy_probe_url)',
     'enable_mdns%' : '<(enable_mdns)',
     'enable_enhanced_bookmarks%' : '<(enable_enhanced_bookmarks)',
     'enable_hangout_services_extension%' : '<(enable_hangout_services_extension)',
     'v8_optimized_debug%': '<(v8_optimized_debug)',
     'proprietary_codecs%': '<(proprietary_codecs)',
     'use_goma%': '<(use_goma)',
     'gomadir%': '<(gomadir)',
 
-    # Use system nspr instead of the bundled one.
-    'use_system_nspr%': 0,
-
     # Use system protobuf instead of bundled one.
     'use_system_protobuf%': 0,
 
     # Use system yasm instead of bundled one.
     'use_system_yasm%': 0,
 
     # Use system ICU instead of bundled one.
     'use_system_icu%' : 0,
 
     # Default to enabled PIE; this is important for ASLR but we may need to be
@@ skipping 3970 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }