Work on Mac GN build

build/config/clang/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.
 
 declare_args() {
   # Indicates if the build should use the Chrome-specific plugins for enforcing
   # coding guidelines, etc. Only used when compiling with Clang.
   clang_use_chrome_plugins = true
 }
 
@@ skipping 13 matching lines @@
         "//third_party/llvm-build/Release+Asserts/lib/libFindBadConstructs.so",
         root_build_dir) ]
     }
 
     cflags += [
       "-Xclang", "-add-plugin",
       "-Xclang", "find-bad-constructs",
     ]
   }
 }
+
+# Enables some extra Clang-specific warnings. Some third-party code won't
+# compile with these so may want to remove this config.
+config("extra_warnings") {
+  cflags = [
+    "-Wheader-hygiene"
+
+    # Warns when a const char[] is converted to bool.
+    "-Wstring-conversion",
+  ]
+}