Handle unpatched VS xtree header

build/config/win/visual_studio_version.gni

Index: build/config/win/visual_studio_version.gni
diff --git a/build/config/win/visual_studio_version.gni b/build/config/win/visual_studio_version.gni
index 6a2828c355319ec24bcf87aafe70499f599f07df..62730dc4ce49c86ad556221453acc4c8351f66f5 100644
--- a/build/config/win/visual_studio_version.gni
+++ b/build/config/win/visual_studio_version.gni
@@ -20,6 +20,11 @@ declare_args() {
   # This value is the default location, override if you have a different
   # installation location.
   windows_sdk_path = "C:\Program Files (x86)\Windows Kits\8.1"
+
+  # Whether the VS xtree header has been patched to disable warning 4702. If
+  # it has, then we don't need to disable 4702 (unreachable code warning).
+  # The patch is preapplied to the internal toolchain and hence all bots.
+  msvs_xtree_patched = false
 }
 
 if (visual_studio_path == "") {
@@ -37,3 +42,9 @@ if (visual_studio_path == "") {
          "You must set the wdk_path if you set the visual studio path")
   visual_studio_runtime_dirs = []
 }
+
+if (!msvs_xtree_patched) {
+  if (exec_script("../../win_is_xtree_patched.py", [], "value") == 1) {
+    msvs_xtree_patched = true
+  }
+}