Use the hermetic toolchain if FORCE_MAC_TOOLCHAIN is set.

build/toolchain/toolchain.gni

Index: build/toolchain/toolchain.gni
diff --git a/build/toolchain/toolchain.gni b/build/toolchain/toolchain.gni
index 678b91ac84ff75045bd768ceed0b47885d9b7064..7901047a7ffde407c92b73ffd47a85a4a02032e7 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -46,7 +46,13 @@ declare_args() {
   # instead use a hermetic install of Xcode. [The hermetic install can be
   # obtained with gclient sync after setting the environment variable
   # FORCE_MAC_TOOLCHAIN].
-  use_system_xcode = true
+  use_system_xcode = ""
+}
+
+if (is_mac && use_system_xcode == "") {
+  _result =
+      exec_script("//build/mac/should_use_hermetic_xcode.py", [], "list lines")
+  use_system_xcode = _result[0] == "0"

[Dirk Pranke, 2016/10/18 01:51:58]

If you use "value" instead of "list lines" you can write just `_result =
exec_script(...) == "0"`.

[erikchen, 2016/10/18 20:35:39]

Done.

 }
 
 # The path to the hermetic install of Xcode. Only relevant when