GN Mac build work

tools/gn/secondary/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 61 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.
 } 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 282 matching lines @@
     # Special toolchain for ARM cross-compiling.
     set_default_toolchain("//build/toolchain/linux:arm-cross-compile")
   } else {
     # Use whatever GCC is on the current platform.
     set_default_toolchain(host_toolchain)
   }
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 }