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..e70e2a981866aa7d5138296159d08a1dae469215 100644
--- a/build/toolchain/toolchain.gni
+++ b/build/toolchain/toolchain.gni
@@ -46,7 +46,12 @@ 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", [], "value")

[brettw, 2016/10/19 20:38:32]

GN has a "getenv" function. Can you use that here?

[erikchen, 2016/10/19 20:45:06]

How convenient. Done. This also removes the dependency on .gn, and hence the
need for another pass from you. ;)

+  use_system_xcode = _result == 0
 }
 
 # The path to the hermetic install of Xcode. Only relevant when