Handle unpatched VS xtree header

build/config/win/BUILD.gn

 # Copyright (c) 2013 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.
 
 import("//build/config/win/visual_studio_version.gni")
 
 # Compiler setup for the Windows SDK. Applied to all targets.
 config("sdk") {
   # The include path is the stuff returned by the script.
   #include_dirs = msvc_config[0]  TODO(brettw) make this work.
@@ skipping 120 matching lines @@
 
 # Nominmax --------------------------------------------------------------------
 
 # Some third party code defines NOMINMAX before including windows.h, which
 # then causes warnings when it's been previously defined on the command line.
 # For such targets, this config can be removed.
 
 config("nominmax") {
   defines = [ "NOMINMAX" ]
 }
+
+# 4702 warning -----------------------------------------------------------------
+
+# VS xtree header file needs to be patched or 4702 (unreachable code warning)
+# is reported if _HAS_EXCEPTIONS=0. Disable the warning if xtree is not patched.
+
+config("msvs_xtree_unreachable_code") {
+  if (!msvs_xtree_patched) {
+    cflags = [ "/wd4702" ]
+  }
+}