Never use the hermetic xcode toolchain for iOS recipes.

scripts/slave/recipe_modules/chromium/config.py

Index: scripts/slave/recipe_modules/chromium/config.py
diff --git a/scripts/slave/recipe_modules/chromium/config.py b/scripts/slave/recipe_modules/chromium/config.py
index 2c340bf30bd89db9b20eefd0bc299180a3a28485..624cbf39f0f52cbe0c74c0866774ec9b27e84468 100644
--- a/scripts/slave/recipe_modules/chromium/config.py
+++ b/scripts/slave/recipe_modules/chromium/config.py
@@ -63,7 +63,6 @@ def BaseConfig(HOST_PLATFORM, HOST_ARCH, HOST_BITS,
       GYP_MSVS_VERSION = Single(basestring, required=False),
       GYP_USE_SEPARATE_MSPDBSRV = Single(int, jsonish_fn=str, required=False),
       LLVM_DOWNLOAD_GOLD_PLUGIN = Single(int, required=False),
-      FORCE_MAC_TOOLCHAIN = Single(int, required=False),
     ),
     env = ConfigGroup(
       PATH = List(Path),
@@ -236,6 +235,10 @@ def ninja(c):
   c.build_dir = c.CHECKOUT_PATH.join(out_path)
 
 @config_ctx()
+def force_mac_toolchain_off(c):
+  c.env.FORCE_MAC_TOOLCHAIN = 0
+
+@config_ctx()
 def msvs2013(c):
   c.gn_args.append('visual_studio_version=2013')
   c.gyp_env.GYP_MSVS_VERSION = '2013'