Introduce GN to desktop_linux arg and grit_defines list.

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 139 matching lines @@
   is_android = false
   is_chromeos = false
   is_ios = false
   is_linux = true
   is_mac = false
   is_nacl = false
   is_posix = true
   is_win = false
 }
 
+is_desktop_linux = is_linux && !is_chromeos
+
 # =============================================================================
 # CPU ARCHITECTURE
 # =============================================================================
 
 if (is_win) {
   # Always use 32-bit on Windows, even when compiling on a 64-bit host OS,
   # unless the override flag is specified.
   if (force_win64) {
     cpu_arch = "x64"
   } else {
@@ skipping 286 matching lines @@
 } else if (is_linux) {
   host_toolchain = "//build/toolchain/linux:$build_cpu_arch"
   set_default_toolchain("//build/toolchain/linux:$cpu_arch")
 } else if (is_mac) {
   host_toolchain = "//build/toolchain/mac:clang"
   set_default_toolchain(host_toolchain)
 } else if (is_ios) {
   host_toolchain = "//build/toolchain/mac:host_clang"
   set_default_toolchain("//build/toolchain/mac:clang")
 }