Handle unpatched VS xtree header

BUG=

Review URL: https://codereview.chromium.org/965713002

Cr-Commit-Position: refs/heads/master@{#318859}

build/config/compiler/BUILD.gn

@@ -48,6 +48,13 @@ declare_args() {
   # http://gcc.gnu.org/wiki/DebugFission
   use_debug_fission =
       !is_win && use_gold && linux_use_bundled_binutils && !use_ccache
+
+  if (is_win) {
+    # 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
+  }
 }
 
 # default_include_dirs ---------------------------------------------------------
@@ -788,6 +795,17 @@ config("default_warnings") {
       "/wd4610",  # Class can never be instantiated, constructor required.
       "/wd4996",  # Deprecated function 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.
+    if (!msvs_xtree_patched) {
+      if (exec_script("../../win_is_xtree_patched.py", [], "value") == 1) {
+        msvs_xtree_patched = true
+      } else {
+        cflags += [ "/wd4702" ]  # Unreachable code.
+      }
+    }
   } else {
     # Common GCC warning setup.
     cflags = [