[iOS/GN] Allow compilation with system clang.

build/config/clang/clang.gni

Index: build/config/clang/clang.gni
diff --git a/build/config/clang/clang.gni b/build/config/clang/clang.gni
index cb84879466ff0eb77d41ea185c6cc5dd7e5cfccb..610dd4118a66c8fe777706cf8234d33a07d78900 100644
--- a/build/config/clang/clang.gni
+++ b/build/config/clang/clang.gni
@@ -2,8 +2,10 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/toolchain/toolchain.gni")
+
 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 = is_clang && !is_nacl
+  clang_use_chrome_plugins = is_clang && !is_nacl && !use_xcode_clang
 }