Port r282246 to gn.

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 114ce1be29e7a4e8ac218e0aa3c067f3b95c18fc..0eddb2ce95b07aa5719e095abefebdc4e3598123 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -451,6 +451,8 @@ config("runtime_library") {
 # Toggles between higher and lower warnings for code that is (or isn't)
 # part of Chromium.
 
+# TODO: -Werror and /WX should always be on, independent of chromium_code
+# http://crbug.com/393046
 config("chromium_code") {
   if (is_win) {
     cflags = [
@@ -460,7 +462,6 @@ config("chromium_code") {
   } else {
     cflags = [
       "-Wall",
-      "-Werror",
 
       # GCC turns on -Wsign-compare for C++ under -Wall, but clang doesn't,
       # so we specify it explicitly.
@@ -468,6 +469,11 @@ config("chromium_code") {
       # http://code.google.com/p/chromium/issues/detail?id=90453
       "-Wsign-compare",
     ]
+    if (!is_linux) {
+      # TODO: Add this unconditionally once linux builds without warnings with
+      # clang in the gn build.
+      cflags += [ "-Werror" ]
+    }
 
     # In Chromium code, we define __STDC_foo_MACROS in order to get the
     # C99 macros on Mac and Linux.