Convert clang=1 to the GN build.

build/config/BUILDCONFIG.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.
 
 # =============================================================================
 # BUILD FLAGS
 # =============================================================================
 #
 # This block lists input arguments to the build, along with their default
 # values. GN requires listing them explicitly so it can validate input and have
@@ skipping 84 matching lines @@
   is_win = true
 } else if (os == "mac") {
   is_android = false
   is_chromeos = false
   is_ios = false
   is_linux = false
   is_mac = true
   is_nacl = false
   is_posix = true
   is_win = false
-  is_clang = true  # Always use clang on Mac.
+  if (!is_clang) {
+    is_clang = true  # Always use clang on Mac.
+  }
 } else if (os == "android") {
   is_android = false
   is_chromeos = false
   is_ios = false
   is_linux = true
   is_mac = false
   is_nacl = false
   is_posix = true
   is_win = false
 } else if (os == "chromeos") {
@@ skipping 301 matching lines @@
     set_default_toolchain("//build/toolchain/linux:arm")
   } else if (build_cpu_arch == "x86") {
     set_default_toolchain("//build/toolchain/linux:32")
   } else if (build_cpu_arch == "x64") {
     set_default_toolchain("//build/toolchain/linux:64")
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }