Fix build break, set initial value for msan variable.

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 dart_io_support to 1 in Dart's all.gypi or common.gypi,
     # then do not build the native libraries supporting  dart:io.
     'dart_io_support%': 0,
     'dart_io_secure_socket%': 1,
     'asan%': 0,
+    'msan%': 0,
     # Intel VTune related variables.
     'dart_vtune_support%': 0,
     'conditions': [
       ['OS=="linux"', {
         'dart_vtune_root%': '/opt/intel/vtune_amplifier_xe',
       }],
       ['OS=="win"', {
         'dart_vtune_root%': 'C:/Program Files (x86)/Intel/VTune Amplifier XE 2013',
       }],
     ],
@@ skipping 100 matching lines @@
 
       'Dart_Product' : {
         'abstract': 1,
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         }
       },
     },
   },
 }