Allow sysroot to be overridden via GN arguments when using target toolchain.

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.
 
-if (is_android) {
+declare_args() {
+  # The absolute path of the sysroot that is applied when compiling using
+  # the target toolchain.
+  target_sysroot = ""
+}
+
+if (current_toolchain == default_toolchain && target_sysroot != "") {
+  sysroot = target_sysroot
+} else if (is_android) {
   import("//build/config/android/config.gni")
   if (!is_android_webview_build) {
     if (cpu_arch == "x86") {
       sysroot = rebase_path("$android_ndk_root/$x86_android_sysroot_subdir")
     } else if (cpu_arch == "arm") {
       sysroot = rebase_path("$android_ndk_root/$arm_android_sysroot_subdir")
     } else if (cpu_arch == "mipsel") {
       sysroot = rebase_path("$android_ndk_root/$mips_android_sysroot_subdir")
     } else {
       sysroot = ""
@@ skipping 22 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 = ""
 }