gn: declare is_desktop_linux as a build argument

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 13 matching lines @@
   #   0 means no symbols.
   #   -1 means auto-set (off in release, regular in debug).
   symbol_level = -1
 
   # Component build.
   is_component_build = false
 
   # Debug build.
   is_debug = true
 
+  # Whether we're a traditional desktop unix.
+  is_desktop_linux = os == "linux" && os != "chromeos"
+
   # Set to true when compiling with the Clang compiler. Typically this is used
   # to configure warnings.
   is_clang = os == "mac" || os == "ios" || os == "linux" || os == "chromeos"
 
   # Selects the desired build flavor. Official builds get additional
   # processing to prepare for release. Normally you will want to develop and
   # test with this flag off.
   is_official_build = false
 
   # Select the desired branding flavor. False means normal Chromium branding,
@@ skipping 109 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_android) {
   # TODO(cjhopman): enable this assert once bots are updated to not set
   # cpu_arch.
   #assert(cpu_arch == build_cpu_arch, "Android device target architecture should
   #    be set with 'target_arch', not 'cpu_arch'")
   cpu_arch = target_arch
@@ skipping 733 matching lines @@
       }
       if (defined(invoker.sources)) {
         sources = invoker.sources
       }
       if (defined(invoker.visibility)) {
         visibility = invoker.visibility
       }
     }
   }
 }