Handle unpatched VS xtree header

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 5cf0331c87d64bb16123d72e0993db9c448424b1..6593d749e006d3fe3ee0cc7450c85af87898706c 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -48,6 +48,11 @@ declare_args() {
   # http://gcc.gnu.org/wiki/DebugFission
   use_debug_fission =
       !is_win && use_gold && linux_use_bundled_binutils && !use_ccache
+
+  # 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

[brettw, 2015/03/02 19:14:26]

Can you wrap this comment + arg in an is_win conditional so people on other
platforms don't see this variable in their gn args? You need to put the comment
inside the conditional for "gn args" to associated it with the parameter
properly.

if (is_win) {
  # Whether the ...
  msvs_xtree_patch = false
}

 }
 
 # default_include_dirs ---------------------------------------------------------
@@ -775,6 +780,17 @@ 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 = [