build CrOS chrome with gn

build/toolchain/cros/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/toolchain/gcc_toolchain.gni")
 
 declare_args() {
   # The CrOS build system supports many different kinds of targets across
   # many different architectures. Bringing your own toolchain is also supported,
   # so it's actually impossible to enumerate all toolchains for all targets
   # as GN toolchain specifications.
   # These arguments provide a mechanism for specifying your CC, CXX and AR at
   # buildfile-generation time, allowing the CrOS build system to always use
   # the right tools for the current target.
   cros_target_cc = ""
   cros_target_cxx = ""
   cros_target_ar = ""
 }
 
-gcc_toolchain("target") {
-  assert(cros_target_cc != "", "Must provide target CC.")
-  assert(cros_target_cxx != "", "Must provide target CXX.")
-  assert(cros_target_ar != "", "Must provide target AR.")
+clang_toolchain("clang_x64") {
+  toolchain_cpu = "x64"
+  toolchain_os = "linux"
+}
 
-  cc = "${cros_target_cc}"
-  cxx = "${cros_target_cxx}"
+gcc_toolchain("x64") {
+  # These defaults permit building on a Linux host.
+  cc = "gcc"
+  cxx = "g++"
+  ar = "ar"
 
-  ar = "${cros_target_ar}"
+  # But to build for a specific board, the cros_* args will need to be defined.
+  if (cros_target_cc != "") {
+    cc = "${cros_target_cc}"
+  }
+  if (cros_target_cxx != "") {
+    cxx = "${cros_target_cxx}"
+  }
+  if (cros_target_ar != "") {
+    ar = "${cros_target_ar}"
+  }
   ld = cxx
 
-  toolchain_cpu = "${target_cpu}"
+  toolchain_cpu = "x64"
   toolchain_os = "linux"
   is_clang = is_clang
   use_ccache = false
-  use_goma = false
 }
+
+clang_toolchain("clang_x86") {
+  toolchain_cpu = "x86"
+  toolchain_os = "linux"
+}
+
+gcc_toolchain("x86") {
+  cc = "gcc"
+  cxx = "g++"
+
+  readelf = "readelf"
+  nm = "nm"
+  ar = "ar"
+  ld = cxx
+
+  # To build for a specific board, the cros_* args will need to be defined.
+  if (cros_target_cc != "") {
+    cc = "${cros_target_cc}"
+  }
+  if (cros_target_cxx != "") {
+    cxx = "${cros_target_cxx}"
+  }
+  if (cros_target_ar != "") {
+    ar = "${cros_target_ar}"
+  }
+
+  toolchain_cpu = "x86"
+  toolchain_os = "linux"
+  is_clang = false
+}