Change GN defaults for toolchains for CrOS.

build/config/sysroot.gni

 # 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.
 
 # This header file defines the "sysroot" variable which is the absolute path
 # of the sysroot. If no sysroot applies, the variable will be an empty string.
 
 import("//build/config/chrome_build.gni")
 
 declare_args() {
   # The absolute path of the sysroot that is applied when compiling using
-  # the target toolchain.
+  # the target toolchain. If target_sysroot is set, it will override the
+  # effect of use_sysroot; this allows you to use both target_sysroot
+  # and use_sysroot to set one sysroot for the target and turn off
+  # sysroots for the other toolchains.
   target_sysroot = ""
   use_sysroot = true
 }
 
 if (current_os == target_os && current_cpu == target_cpu &&
     target_sysroot != "") {
   sysroot = target_sysroot
 } else if (is_android) {
   import("//build/config/android/config.gni")
   if (current_cpu == "x86") {
@@ skipping 48 matching lines @@
   }
 } else if (is_mac) {
   import("//build/config/mac/mac_sdk.gni")
   sysroot = mac_sdk_path
 } else if (is_ios) {
   import("//build/config/ios/ios_sdk.gni")
   sysroot = ios_sdk_path
 } else {
   sysroot = ""
 }