Disable NSS build targets in a Dartium build.

runtime/tools/gyp/runtime-configurations.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 {
   'variables': {
     'dart_debug_optimization_level%': '2',
+    # If we have not set in_dartium to 0 in Dart's all.gypi or common.gypi,
+    # then we must be in Dartium, using its global files instead.
+    'in_dartium%': 1,
   },
 
   'target_defaults': {
     'configurations': {
 
       'Dart_Base': {
         'abstract': 1,
         'xcode_settings': {
           'GCC_WARN_HIDDEN_VIRTUAL_FUNCTIONS': 'YES', # -Woverloaded-virtual
         },
@@ skipping 39 matching lines @@
 
       'Dart_Release': {
         'abstract': 1,
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }