Remove Chrome OS-related GN build configuration

build/config/linux/BUILD.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.
 
 import("//build/config/features.gni")
 import("//build/config/sysroot.gni")
 import("//build/config/ui.gni")
 import("//tools/generate_library_loader/generate_library_loader.gni")
 
 config("sdk") {
   if (sysroot != "") {
     cflags = [ "--sysroot=" + sysroot ]
     ldflags = [ "--sysroot=" + sysroot ]
 
     # Need to get some linker flags out of the sysroot.
     ldflags += [ exec_script("sysroot_ld_path.py",
                              [
                                rebase_path("//build/linux/sysroot_ld_path.sh",
                                            root_build_dir),
                                sysroot,
                              ],
                              "value") ]
   }
-
-  # Set here because OS_CHROMEOS cannot be autodetected in build_config.h like
-  # OS_LINUX and the like.
-  if (is_chromeos) {
-    defines = [ "OS_CHROMEOS" ]
-  }
 }
 
 config("x11") {
   libs = [
     "X11",
     "Xi",
   ]
 }
 
 config("xcomposite") {
@@ skipping 81 matching lines @@
 
   functions = libudev_functions
 }
 
 group("udev") {
   deps = [
     ":udev0_loader",
     ":udev1_loader",
   ]
 }