Use target_sysroot when building for target OS on target CPU

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.
   target_sysroot = ""
   use_sysroot = true
 }
 
-if (current_toolchain == default_toolchain && target_sysroot != "") {
+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") {
     sysroot = "$android_ndk_root/$x86_android_sysroot_subdir"
   } else if (current_cpu == "arm") {
     sysroot = "$android_ndk_root/$arm_android_sysroot_subdir"
   } else if (current_cpu == "mipsel") {
     sysroot = "$android_ndk_root/$mips_android_sysroot_subdir"
   } else if (current_cpu == "x64") {
     sysroot = "$android_ndk_root/$x86_64_android_sysroot_subdir"
   } else if (current_cpu == "arm64") {
     sysroot = "$android_ndk_root/$arm64_android_sysroot_subdir"
   } else if (current_cpu == "mips64") {
     sysroot = "$android_ndk_root/$mips64_android_sysroot_subdir"
   } else {
     sysroot = ""
   }
-} else if (is_linux && !is_chromeos && use_sysroot) {

[hashimoto, 2016/04/20 05:28:58]

This !is_chromeos has nothing to do with the problem being fixed, but I'm just
removing it now as it's redundant.

IIUC this !is_chromeos was introduced in
https://codereview.chromium.org/115643002, but it's unneeded.
When building for real Chrome OS devices, we always set target_sysroot so we
never reach here.
When building Linux Chrome with target_os="chromeos" (this is only used by UI
developers), we should just do the same thing as Linux Chrome.

[hashimoto, 2016/04/21 00:54:03]

Ugh, this !is_chromeos is actually needed because debian sysroot is too old to
build Chrome OS code.
This means we rely on developers to have sufficiently new header files in their
workstations, but it shouldn't matter much as this path will be only used by UI
developers.
Reverted this part.

+} else if (is_linux && use_sysroot) {
   # By default build against a sysroot image downloaded from Cloud Storage
   # during gclient runhooks.
   if (current_cpu == "x64") {
     sysroot = "//build/linux/debian_wheezy_amd64-sysroot"
   } else if (current_cpu == "x86") {
     sysroot = "//build/linux/debian_wheezy_i386-sysroot"
   } else if (current_cpu == "mipsel") {
     sysroot = "//build/linux/debian_wheezy_mips-sysroot"
   } else if (current_cpu == "arm") {
     sysroot = "//build/linux/debian_wheezy_arm-sysroot"
@@ skipping 23 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 = ""
 }