Enabling /ZW (CompileAsWinRT) option for msvs

test/win/compiler-flags/compile-as-winrt.gyp

+# Copyright (c) 2016 Google Inc. 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': 'test-compile-as-winrt',
+      'type': 'executable',
+      'msvs_windows_sdk_version': 'v10.0',
+      'msvs_settings': {
+        'VCCLCompilerTool': {
+          'AdditionalUsingDirectories': ['$(VCInstallDir)vcpackages;$(WindowsSdkDir)UnionMetadata;%(AdditionalUsingDirectories)'],
+          'CompileAsWinRT': 'true'
+        }
+      },
+      'sources': ['compile-as-winrt.cc']
+    }
+  ]
+}