Use target_sysroot when building for target OS on target CPU

build/config/sysroot.gni

Index: build/config/sysroot.gni
diff --git a/build/config/sysroot.gni b/build/config/sysroot.gni
index dcd215a345c7267ac9f0172fefe2a698c3380ea6..eb6f1b42a133fe0b79737c2448db2d307b7eceda 100644
--- a/build/config/sysroot.gni
+++ b/build/config/sysroot.gni
@@ -14,7 +14,8 @@ declare_args() {
   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")
@@ -33,7 +34,7 @@ if (current_toolchain == default_toolchain && target_sysroot != "") {
   } 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") {