Fix build break, set initial value for msan variable.

tools/gyp/all.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.
 
 # A set of variables needed to build some of the Chrome based subparts of the
 # Dart project. This is in no way a complete list of variables being defined
 # by Chrome, but just the minimally needed subset.
 {
   'variables': {
     'library': 'static_library',
     'component': 'static_library',
     'target_arch': 'ia32',
     # Flag that tells us whether to build native support for dart:io.
     'dart_io_support': 1,
     # Flag that tells us whether this is an ASAN build.
     'asan%': 0,
+    # Flag that tells us whether this is a MSAN build.
+    'msan%': 0,
   },
   'conditions': [
     [ 'OS=="linux"', {
       'target_defaults': {
         'ldflags': [ '-pthread', ],
       },
     }],
     [ 'OS=="win"', {
       'target_defaults': {
         'msvs_cygwin_dirs': ['<(DEPTH)/third_party/cygwin'],
       },
       'includes': [
         'msvs.gypi',
       ],
     }],
     [ 'OS=="mac"', {
       'includes': [
         'xcode.gypi',
       ],
     }],
   ],
   'includes': [
     'configurations.gypi',
   ],
 }