Correct iOS GN hermetic support.

ios/build/tools/setup-gn.py

Index: ios/build/tools/setup-gn.py
diff --git a/ios/build/tools/setup-gn.py b/ios/build/tools/setup-gn.py
index 1453fa546bc4aa72e862bdd9d5ad3ebaa4865ba9..b4a53006354bb71140fe89383d5170def9f6de44 100755
--- a/ios/build/tools/setup-gn.py
+++ b/ios/build/tools/setup-gn.py
@@ -96,6 +96,8 @@ class GnGenerator(object):
     args.append(('is_official_build', self._config == 'Official'))
     args.append(('is_chrome_branded', 'is_official_build'))
     args.append(('use_xcode_clang', 'is_official_build'))
+    if os.environ['FORCE_MAC_TOOLCHAIN']:
+      args.append(('use_system_xcode', False))

[justincohen, 2016/10/26 00:36:48]

I know we want to get rid of env var, but for now we still have this.  wdyt?

[sdefresne, 2016/10/26 15:33:07]

The bots don't use the output of this script and developer can configure
variables via ~/.setup-gn, so I think we don't need this change. You can just
have the following to control the same thing without using environment
variables:

$ cat ~/.setup-gn
[gn_args]
use_system_xcode = false

WDYT?

[justincohen, 2016/10/27 13:36:39]

Per offline discussion with sdefresne@, I will leave this here for now.  When we
remove the environment
variable FORCE_MAC_TOOLCHAIN, this will be removed as well.

 
     cpu_values = self.TARGET_CPU_VALUES[self._target]
     build_arch = self._settings.getstring('build', 'arch')